Low cost and high performance bolometer circuitry and methods

ABSTRACT

A bolometer circuit may include an active bolometer configured to receive external infrared (IR) radiation and a resistive load, which are configured to be connected in series in a bolometer conduction path from a supply voltage node to a common voltage node. A node in the bolometer conduction path between the resistive load and the active bolometer is coupled to a first input of an op-amp. A variable voltage source is coupled to a second input of the op-amp to provide a reference voltage level. The op-amp maintains the reference voltage level at the first input to generate a current flow in response to a resistance change of the active bolometer due to the external IR radiation. The amplifier circuit may be configured as a feedback amplifier or an integrating amplifier. The bolometer circuit may be configured to enable a low-power mode of operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/US2015/056108 filed Oct. 16, 2015 and entitled “LOW COST ANDHIGH PERFORMANCE BOLOMETER CIRCUITRY AND METHODS,” which is incorporatedherein by reference in its entirety.

International Patent Application No. PCT/US2015/056108 filed Oct. 16,2015 claims priority to and the benefit of U.S. Provisional PatentApplication No. 62/242,941 filed on Oct. 16, 2015 and entitled “LOW COSTAND HIGH PERFORMANCE BOLOMETER CIRCUITRY AND METHODS,” which is herebyincorporated by reference in its entirety.

International Patent Application No. PCT/US2015/056108 filed Oct. 16,2015 also claims priority to and the benefit of U.S. Provisional PatentApplication No. 62/064,964 filed on Oct. 16, 2014 and entitled “LOW COSTAND HIGH PERFORMANCE BOLOMETER CIRCUITRY AND METHODS,” which is herebyincorporated by reference in its entirety.

International Patent Application No. PCT/US2015/056108 filed Oct. 16,2015 also claims priority to and the benefit of U.S. Provisional PatentApplication No. 62/064,971 filed on Oct. 16, 2014 and entitled“BOLOMETER CIRCUITRY AND METHODS FOR DIFFERENCE IMAGING,” which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

One or more embodiments of the invention relate generally to thermalimaging devices and more particularly, for example, to bolometercircuitry and related processing.

BACKGROUND

A bolometer, which changes its resistance in response to opticalheating, is often used in the art to detect the intensity of incidentinfrared (IR) radiation or to capture multi-pixel IR images of incidentIR radiation intensity. Typically, to measure incident IR radiation, abias (e.g., a bias voltage or current) is applied across a bolometer sothat its resistance, or any change thereof, can be measured andtranslated into a signal indicative of the intensity of IR radiationreceived at the bolometer. In this regard, many conventional bolometercircuits include a significant amount of circuitry dedicated togenerating and controlling the bias across bolometers at a desiredlevel. For example, conventional bias generation and control circuitsmay include transistors or other control mechanisms in a bolometerconduction path, along with circuitry to drive such transistors or othercontrol mechanisms, to generate and maintain a desired bias acrossbolometers in face of variations or mismatches in operatingcharacteristics of components and changing ambient or internalconditions.

Unfortunately, such additional transistors or other control mechanismsin a bolometer conduction path not only increase the size, cost,complexity, and power consumption of conventional bolometer circuits,but also introduce noise in the sensitive bolometer conduction path.Furthermore, because such additional transistors or other controlmechanisms may control the bias by limiting the current flow and/orvoltage on the bolometer conduction path, the voltage and/or currentavailable for biasing the bolometer is effectively reduced to only aportion of the supply voltage or current, which in turn reduces thesensitivity of conventional bolometer circuits. Even with such costlybias generation and control approaches, conventional bolometer circuitstypically exhibit a reduced usable signal range for representing IRradiation intensity (e.g., reduced signal swing) because of the need toallow for effects on output signals due to changing ambient and internalconditions such as, for example, self-heating of bolometers (alsoreferred to as pulsed bias heating or pulse bias heating) or otherfactors. Accordingly, there is a need for a high-performance bolometercircuit that generates and maintains biases across bolometers at adesired level and/or provides a large usable signal range without thecost, size, complexity, noise, and power consumption associated withconventional bolometer circuits.

SUMMARY

Various techniques are disclosed for bolometer circuits and methods toprovide a desired bias across bolometers to generate an output signal.For example, in one embodiment, a bolometer circuit may include asubstrate; an active bolometer configured to receive external infrared(IR) radiation and substantially thermally isolated from the substrate;a resistive load, wherein the active bolometer and the resistive loadare configured to be connected in series in a bolometer conduction pathfrom a supply voltage node to a common voltage node; an amplifiercircuit comprising an operational amplifier (op-amp) having a firstinput coupled to a node in the bolometer conduction path between theresistive load and the active bolometer; and a variable voltage sourcecoupled to a second input of the op-amp to provide a reference voltagelevel, wherein the amplifier circuit is configured to generate a currentflow to the amplifier circuit in response to a resistance change of theactive bolometer due to the external IR radiation by maintaining thereference voltage level at the first input of the op-amp, and whereinthe amplifier circuit is further configured to convert the current flowinto an output voltage at an output of the op-amp that is indicative ofan intensity of the external IR radiation received at the activebolometer.

The bolometer circuit, according to one or more embodiments, may beimplemented in a focal plane array (FPA) having multiple rows andcolumns of the active bolometers to generate an output signalrepresenting a multi-pixel IR image using suitable time-multiplexingtechniques. A sequence of such IR image frames may also be generated insome embodiments.

In some embodiments, the resistive load comprises a thermally shortedbolometer and a transistor (e.g., a MOSFET) connected in series tooperate as a current source that generates a load current to the activebolometer. In some embodiments, a resistance of the thermally shortedbolometer is larger than a resistance of the active bolometer, and thesupply voltage node provides a supply voltage level that is higher thana nominal voltage level applicable to generate the load current in anominal case of the thermally shorted bolometer having a resistancesimilar to the resistance of the active bolometer. In other embodiments,a resistance of the resistive load is similar to a resistance of theactive bolometer.

In some embodiments, the amplifier circuit comprises a resistive gaincoupling the output of the op-amp to the first input of the op-amp toconfigure the amplifier circuit as a feedback amplifier, the resistivegain comprises a thermally shorted bolometer that is thermally shortedto the substrate to operate as a temperature-compensated gain for theamplifier circuit, and the amplifier circuit is configured to maintain,in response to the reference voltage level, a bias voltage across theactive bolometer and a load bias voltage across the resistive load viathe first input of the op-amp coupled to the bolometer conduction pathnode, and generate the output voltage in response to the current flowflowing through the resistive gain. The current flow to the amplifiercircuit is generated in response to a difference between the currentgenerated by the load bias voltage being applied to the resistive loadand the current generated by the bias voltage being applied to theactive bolometer that exhibits the resistance change due to the externalIR radiation

In other embodiments, the amplifier circuit further comprises acapacitor coupling an output of the op-amp to the first input of theop-amp to configure the amplifier circuit as an integrating amplifier, abuffer connected to the bolometer conduction path node, and a resistorconnected to the first input of the op-amp, the buffer and the resisterbeing connected in series to couple the bolometer conduction path nodeto the first input of the op-amp. The amplifier circuit is configured tomaintain the reference voltage level at one end of the resistorconnected to the first input of the op-amp, and the other end of theresistor is configured to receive, via the buffer, a voltage level atthe bolometer conduction path node set in response to the load currentflowing through the active bolometer that exhibits the resistance changedue to the external IR radiation. The voltage difference between the twoends of the resistor generates the current flow to the amplifier, andthe amplifier circuit is configured to integrate the current flow by thecapacitor to generate the output voltage.

In some embodiments, the bolometer circuit is configured to operate in anormal mode or in a low-power mode based on selective opening or closingof switches associated with the bolometer conduction path and theamplifier circuit, where in the normal mode, the output of the op-ampprovides the output voltage indicative of the intensity of the externalIR radiation received at the active bolometer and in the low-power mode,the output of the op-amp is driven to a predetermined voltage level andthe op-amp, the bolometer conduction path, or both are disconnected frompower.

According to one or more embodiments, a bolometer circuit implemented ina FPA may comprise a plurality of the active bolometers arranged in aFPA having columns and rows, a plurality of column circuits eachassociated with a column of the bolometer FPA and each comprising theresistive load, the amplifier circuit, and the switches associated withthe bolometer conduction paths and the amplifier circuit, and a controlcircuit configured to control the switches to operate the bolometer FPAin a normal imaging mode or a low-power detection mode, wherein in thenormal imaging mode, all columns of the bolometer FPA operate in thenormal mode, and wherein in the low-power detection mode, some columnsof the bolometer FPA operate in the normal mode while the remainder ofthe columns of the bolometer FPA operate in the low-power mode. In someembodiments, the control circuit may be further configured to detect achange in the external IR radiation using those columns of the bolometerFPA that operate in the normal mode while operating the bolometer FPA inthe low-power detection mode and switch from the low-power detectionmode to the normal imaging mode for the bolometer FPA in response todetecting the change in the external IR radiation. In some embodiments,those columns that operate in the normal mode are selected in around-robin manner from all columns of the bolometer FPA as each IRimage frame is captured, so as to reduce a burn-in effect on the columnsof the bolometer FPA.

In one embodiment, a bolometer circuit may include a substrate; anactive bolometer configured to receive external infrared (IR) radiationand substantially thermally isolated from the substrate; a firstresistive element, wherein the active bolometer and the first resistiveelement are configured to be connected in series in a conduction pathfrom a supply voltage node to a common voltage node, and wherein thefirst resistive element provides a resistive load for the bolometer; anamplifier having a first input connected to a node between the firstresistive element and the active bolometer in the conduction path; asecond resistive element coupling an output of the amplifier to thefirst input to configure the amplifier as a feedback amplifier; and avariable voltage source coupled to a second input of the amplifier toprovide a reference voltage level in response to which the amplifiermaintains a bias across the bolometer via the first input and producesat the output an output signal indicative of an intensity of theexternal IR radiation received at the active bolometer.

In another embodiment, the first resistive element may be a firstbolometer thermally shorted to the substrate to provide atemperature-compensated resistive load for the active bolometer. Inanother embodiment, the second resistive element may be a secondbolometer thermally shorted to the substrate to provide atemperature-compensated resistive gain for the amplifier. The bolometercircuit may also include a low-pass filter (LPF) to reduce noise fromthe output signal and/or a sample-and-hold circuit to sample the outputsignal, depending on embodiments. For some embodiments, the variablevoltage source may be implemented using a digital-to-analog converter(DAC) configured to generate the reference voltage level in response tobias adjustment bits. For some embodiments, the variable voltage sourcemay be implemented using a reference conduction path comprising a thirdresistive element and a blind bolometer in a series connection mirroringthe active bolometer conduction path to track resistance changes due toself-heating of the active bolometer. The bolometer circuit according tosome embodiments may allow additional bias adjustment to be made viaoffset adjustment circuitry provided in one or more parts of thebolometer circuit. In various embodiments, the bolometer circuit may beimplemented in a focal plane array comprising a readout integratedcircuit (ROIC) to generate an output signal representing a multi-pixelIR image using suitable time-multiplexing techniques.

In a further embodiment, a method of generating an output signal in abolometer circuit includes selectively connecting an active bolometer toa resistive load in series to form a bolometer conduction path from asupply voltage node to a common voltage node, wherein the activebolometer is configured to receive external infrared (IR) radiation andsubstantially thermally isolated from a substrate to which the activebolometer is attached; providing a reference voltage level to anoperational amplifier (op-amp) that has a first input coupled to a nodein the bolometer conduction path between the resistive element and theactive bolometer, wherein the reference voltage level is received via asecond input of the op-amp; generating a current flow to the op-amp inresponse to a resistance change of the active bolometer due to theexternal IR radiation by maintaining the reference voltage level at thefirst input of the op-amp; and converting the current flow into anoutput voltage at an output of the op-amp that is indicative of anintensity of the external IR radiation received at the active bolometer.

The method according to one or more embodiments may be performed withrespect to a focal plane array (FPA) having multiple rows and columns ofthe active bolometers to generate an output signal representing amulti-pixel IR image using suitable time-multiplexing techniques. Asequence of such IR image frames may also be generated in someembodiments.

In some embodiments, the method further comprises selectively operatingthe bolometer circuit in a normal mode or in a low-power mode byselectively opening or closing switches associated with the bolometerconduction path and the op-amp, wherein in the normal mode, the outputof the op-amp provides the output signal indicative of the intensity ofthe external IR radiation received at the active bolometer, and in thelow-power mode, the output of the op-amp is driven to a predeterminedvoltage level and the op-amp, the bolometer conduction path, or both aredisconnected from power.

According to one or more embodiments, the method performed with respectto a FPA of bolometers may further include selectively operating thebolometer FPA in a normal imaging mode or a low-power detection mode,where in the normal imaging mode, all columns of the bolometer FPAoperate in the normal mode, and in the low-power detection mode, somecolumns of the bolometer FPA operate in the normal mode while theremainder of the columns of the bolometer FPA operate in the low-powermode. In some embodiments, the method may further include detecting achange in the external IR radiation using those columns of the bolometerFPA that operate in the normal mode while operating the bolometer FPA inthe low-power detection mode, and switching from the low-power detectionmode to the normal imaging mode for the bolometer FPA in response todetecting the change in the external IR radiation. In some embodiments,the method may further include capturing, using the bolometer FPA, asequence of IR image frames representing the external IR radiation, andwhile operating the bolometer FPA in the low-power detection mode,selecting those columns that operate in the normal mode in a round-robinmanner from all columns of the bolometer FPA as each IR image frames iscaptured, so as to reduce a burn-in effect on the columns of thebolometer FPA.

In some embodiments, the method may further include generating a loadcurrent to the active bolometer by the resistive load that comprises athermally shorted bolometer (which may have a larger resistance than theactive bolometer) and a transistor connected in series to operate as acurrent source, and supplying a supply voltage level that is higher thana nominal voltage level applicable to generate the load current in anominal case of the resistive load having a resistance similar to theresistance of the active bolometer.

In some embodiments, the first input of the op-amp is coupled to theoutput of the op-amp via a resistive gain to form a feedback amplifierconfiguration; the resistive gain comprises a thermally shortedbolometer that is thermally shorted to the substrate; and the methodfurther comprises maintaining, in response to the reference voltagelevel, a bias voltage across the active bolometer and a load biasvoltage across the resistive load via the first input of the op-ampcoupled to the bolometer conduction path node. In such embodiments, thegenerating of the current flow comprises generating the current flow inresponse to a difference between the current generated by the load biasvoltage being applied to the resistive load and the current generated bythe bias voltage being applied to the active bolometer that exhibits theresistance change due to the external IR radiation, and the convertingof the current flow comprises generating the output voltage in responseto the current flow flowing through the resistive gain.

In other embodiments, the first input of the op-amp is coupled to theoutput of the op-amp via a capacitor to form an integrating amplifierconfiguration; the bolometer conduction path node is coupled to thefirst input of the op-amp via a buffer and a resister connected inseries; and the method further comprises setting a voltage level at thebolometer conduction path node in response to the load current flowingthrough the active bolometer that exhibits the resistance change due tothe external IR radiation, receiving the voltage level by the buffer topass the voltage level to one end of the resistor, and maintaining thereference voltage level at the other end of the resistor. In suchembodiments, the generating of the current flow comprises generating thecurrent flow in response to a difference between the voltage level atthe one end and the reference voltage level at the other end of theresistor, and the converting of the current flow comprises integratingthe current flow to the capacitor to generate the output voltage.

In one embodiment, a method of biasing an active bolometer to generatean output signal includes selectively connecting the active bolometer toa first resistive element in series to form a conduction path, whereinthe active bolometer is configured to receive external IR radiation andsubstantially thermally isolated from a substrate to which the activebolometer is attached; providing a reference voltage to an amplifierhaving a first input connected to a node between the first resistiveelement and the bolometer in the conduction path, wherein the referencevoltage is received via a second input of the amplifier, and wherein thefirst input is coupled to an output of the amplifier via a secondresistive element to form a feedback amplifier configuration; biasingone end of the bolometer and one end of the first resistive element witha voltage at the node that tracks the reference voltage due to thefeedback amplifier configuration; and converting a current flow throughthe second resistive element into the output signal at the amplifieroutput, wherein the current flow is generated in response to the biasingand a resistance change of the bolometer due at least in part to theexternal IR radiation. In yet further embodiments, the method mayinclude various additional features, variations, or modifications inaccordance with various techniques discussed herein in connection withthe bolometer circuit.

Moreover, various techniques are disclosed for bolometer circuits andrelated methods for thermal imaging in a difference domain, where eachpixel value represents a difference in incident IR radiation intensitybetween adjacent infrared detectors. For example, in one embodiment, abolometer circuit may include an array of bolometers each configured togenerate a pixel signal in response to a bias applied and incidentinfrared (IR) radiation received at the each bolometer, wherein eachcolumn of the array of bolometers comprises an amplifier having an inputand an output, a first plurality of switches each configured toselectively provide a supply voltage to a respective one of bolometersof the each column, a second plurality of switches each configured toselectively route a difference of the pixel signals of a respectiveadjacent pair of the bolometers of the each column to the input of theamplifier, and a third plurality of switches configured to selectivelyprovide a common voltage to a respective one of the bolometers of theeach column.

The bolometer circuit may include a control circuit configured togenerate control signals for the switches. For example, the controlcircuit may be configured to generate control signals to close one ofthe first plurality of switches, one of the second plurality ofswitches, and one of the third plurality of switches while opening theremainder of the first, second, and third pluralities of switches, suchthat biases are applied to a selected adjacent pair of the bolometers ofeach column and a difference signal representative of the difference ofthe pixel signals for the selected adjacent pair is generated at theoutput of the amplifier.

The control circuit according to various embodiments may be configuredto generate control signals to selectively open or close the switches toobtain various types of difference frames (or difference image frames)that comprise rows of such difference signals. For example, in oneembodiment, the control circuit may be configured to repeat generatingthe control signals to sequentially obtain even difference signals for afirst plurality of adjacent pairs of the bolometers of each column, theeven difference signals corresponding to those difference signals thatare obtained by subtracting the pixel signals of odd rows from the pixelsignals of corresponding even rows in the first plurality of adjacentpairs of the bolometers; and further configured to repeat generating thecontrol signals to sequentially obtain odd difference signals for asecond plurality of adjacent pairs of the bolometers of each column, theodd difference signals corresponding to those difference signals thatare obtained by subtracting the pixel signals of even rows from thepixel signals of corresponding odd rows in the second plurality ofadjacent pairs of the bolometers. As such, the control circuit may beconfigured to selectively open or close the switches according to aspecified timing so that even and odd difference frames may be obtainedby the bolometer circuit. In some embodiments, the control circuit maybe configured to repeat generating the control signals to obtainadditional even and odd rows of difference signals in an oppositedirection.

In some embodiments, the bolometer circuit may comprise a processor(provided as part of a focal plane array of bolometers or providedexternally, for example, in a host device in which the bolometer circuitmay be implemented) configured to combine the even difference signalsand the odd difference signals to generate a difference image comprisingboth even and odd rows of difference signals, and to reconstruct thedifference image into a direct image by cumulatively adding thedifference rows of the difference image. Thus, in such embodiments,signals in a difference domain may be converted into a direct IR imagewhere each pixel value corresponds to IR radiation intensity received ateach detector.

In some embodiments, the bolometer circuit may comprise one or more rowsof blind bolometers substantially shielded from the incident IRradiation, where in the one or more rows of blind bolometers areselectively connectable to a corresponding one or more rows of the arrayof bolometers to provide a pixel signal representing a reference IRintensity level. The blind bolometer row or rows may be used to obtainreference measurement signals (or absolute measurement signals) usefulfor reconstructing direct images and other purposes. For example, in oneembodiment, the control circuit may be further configured to generateadditional control signals to selectively connect the one or more rowsof blind bolometers to the corresponding one or more rows of bolometersto obtain corresponding one or more rows of reference measurementsignals, the reference signals representing differences between thepixel signals of the blind bolometers providing the reference IRintensity levels and the pixels signals of the corresponding bolometers.

In some embodiments, the bolometer circuit may comprise a transimpedancefeedback amplifier and associated circuits configured to set andmaintain the biases across the bolometers without the complexity, cost,size, power consumptions, and noise associated with conventional biascircuitry techniques for bolometer circuits.

According to another embodiment of the disclosure, a method may includereceiving an even difference image frame and an odd difference imageframe, wherein the even difference image frame comprises even rows ofdifference data representative of infrared (IR) radiation intensityreceived at even rows of a bolometer array less IR radiation intensityreceived at respective adjacent odd rows of the bolometer array, andwherein the odd difference image frame comprises odd rows of differencedata representative of IR radiation intensity received at odd rows of abolometer array less IR radiation intensity received at respectiveadjacent even rows of the bolometer array; combining the even and theodd difference image frames to generate a composite difference imagecomprising both the even and the odd rows of difference data; andgenerating a direct image from the composite difference image bycumulatively adding the difference rows from top to bottom or bottom totop to generate rows of the direct image. In one embodiment, the evenand the odd difference image frames may be generated by the variousembodiments of the bolometer circuit disclosed herein.

In some embodiments, various column, row, and/or pixel noise reductionfilters may be applied to the difference image frames and/or thecomposite difference image. Various other novel noise reductiontechniques may be included. For example, in one embodiment, the methodmay further include comparing local smoothness between the compositedifference image and the generated direct image to identify local areasin the generated direct image that exhibit more noise, and applyingcolumn, row, and/or pixel noise reduction filters to the generateddirect image in response to the identified local areas. In anotherexample according to one embodiment, the method may further includecomparing one or more absolute measurement rows (e.g., those thatcontain absolute measurement signals) with corresponding one or morerows of the generated direct image to identify residual spatial noise inthe generated direct image. In yet another example according to oneembodiment, the method may further include comparing one or moreabsolute measurement rows with corresponding one or more rows of thegenerated direct image to determine statistical metrics relating toresidual noise in the generated direct image.

The scope of the invention is defined by the claims, which areincorporated into this section by reference. A more completeunderstanding of embodiments of the invention will be afforded to thoseskilled in the art, as well as a realization of additional advantagesthereof, by a consideration of the following detailed description of oneor more embodiments. Reference will be made to the appended sheets ofdrawings that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a bolometer circuit having anarray of bolometers in accordance with an embodiment of the disclosure.

FIG. 2A illustrates a bolometer circuit to provide a desired level ofbias to a bolometer in accordance with an embodiment of the disclosure.

FIG. 2B illustrates a bolometer circuit to provide a desired level ofbias to a bolometer and to enable a low-power mode of operation inaccordance with an embodiment of the disclosure.

FIG. 3 illustrates a bolometer circuit to provide a desired level ofbias to a plurality of bolometers in accordance with an embodiment ofthe disclosure.

FIG. 4A illustrates a bolometer circuit to provide a desired level ofbias to a plurality of bolometers and to compensate for bolometerself-heating, in accordance with an embodiment of the disclosure.

FIG. 4B illustrates a bolometer circuit to provide a desired level ofbias to a plurality of bolometers and to compensate for bolometerself-heating in accordance with another embodiment of the disclosure.

FIG. 4C illustrates a bolometer circuit to provide a desired level ofbias to a plurality of bolometers and to compensate for bolometerself-heating in accordance with another embodiment of the disclosure.

FIG. 4D illustrates a bolometer circuit comprising the bolometer circuitof FIG. 2B and enabling a low-power detection mode of operation using aplurality of bolometers in accordance with an embodiment of thedisclosure.

FIG. 5A illustrates a bolometer circuit to obtain difference images inaccordance with an embodiment of the disclosure.

FIGS. 5B through 5D show block diagrams illustrating various blindbolometer arrangements for the bolometer circuit of FIG. 5A, inaccordance with various embodiments of the disclosure.

FIG. 6A illustrates a portion of a bolometer circuit to obtaindifference images in accordance with an embodiment of the disclosure.

FIG. 6B illustrates a portion of a bolometer circuit to obtaindifference signals between adjacent columns in accordance with anembodiment of the disclosure.

FIGS. 7A through 7D show block diagrams illustrating how variousdifference frames are captured using the bolometer circuit of FIG. 5A,in accordance with an embodiment of the disclosure.

FIG. 8 illustrates a bolometer circuit to obtain difference images inaccordance with another embodiment of the disclosure.

FIG. 9 illustrates a flowchart of a process to generate direct imagesfrom difference frames captured by the bolometer circuit of FIG. 5A orFIG. 8, in accordance with an embodiment of the disclosure.

FIG. 10 illustrates a flowchart of a noise reduction process that may beperformed on difference frames, in accordance with an embodiment of thedisclosure.

FIG. 11 illustrates a flowchart of a noise reduction process that may beperformed on composite difference images, in accordance with anembodiment of the disclosure.

FIG. 12 illustrates a flowchart of a noise reduction process that may beperformed on reconstructed direct images, in accordance with anembodiment of the disclosure.

FIG. 13 illustrates a thermal imaging module configured to beimplemented in a host device in accordance with an embodiment of thedisclosure.

Embodiments of the invention and their advantages are best understood byreferring to the detailed description that follows. It should beappreciated that like reference numerals are used to identify likeelements illustrated in one or more of the figures.

DETAILED DESCRIPTION

FIG. 1 shows an example bolometer circuit 100 according to an embodimentof the present disclosure. Bolometer circuit 100 comprises a pluralityof active bolometers 102, which may be configured as an array arrangedin a rectangle, square, circle, line or other shape. In somenon-limiting examples, active bolometers 102 may be provided as arectangular array having a dimension of 80×60, 160×120, 320×240, or anyother dimension desired for a particular application. Bolometer circuit100 may comprise various components and circuits, which may becollectively referred to as a Read Out Integrated Circuit (ROIC), thatinterface with active bolometers 102 to generate an output as furtherdescribe herein. Bolometer circuits, such as some embodiments ofbolometer circuit 100, that have an array of active bolometers andassociated circuits formed together on a substrate may also be referredto as focal plane arrays (FPAs).

Some portions (e.g., switches for a particular bolometer) of the ROICmay be associated with and placed in proximity to each particular one ofactive bolometers 102. Each one of active bolometers 102 together withits associated portion of the ROIC may also be referred to as a unitcell. Since active bolometers 102 may be configured as an array,corresponding unit cells may form a unit cell array 104. Also, in thecontext of imaging, each one of active bolometers 102 may also bereferred to as a pixel.

Active bolometers 102 may be provided on a substrate 106, butsubstantially thermally isolated from substrate 106 (e.g., released fromsubstrate 106 such that active bolometers 102 are not substantiallyaffected thermally by substrate 106). Active bolometers 102 may beconfigured to receive infrared radiation from an external scene, forexample, directed onto active bolometers 102 by an optical element suchas an infrared-transmissive lens. Infrared (“IR”) radiation incident onactive bolometers 102 changes the temperature, and therefore theresistivity, of active bolometers 102 as would be understood by oneskilled in the art. The temperature and the resistivity of activebolometers 102 may also track the temperature of substrate 106, butbecause active bolometers 102 are thermally isolated from substrate 106,the rate of temperature change due to substrate is slower than that dueto incident infrared radiation. The ROIC of bolometer circuit 100comprises various components and circuits configured to generate anoutput based on the resistivity changes of active bolometers 102 due toincident infrared radiation.

In one aspect, bolometer circuit 100 may include a bias circuit 108configured to control a bias (e.g., a bias voltage or current) acrossactive bolometers 102 in generating such an output. In general, a biasmay be applied across a bolometer so that the resistance (or any changethereof) of the bolometer can be measured. According to variousembodiments of bolometer circuit 100, bias circuit 108 may be utilizedto control the bias applied across active bolometers 102 to anappropriate level as further described herein, so that the nominalvalues of active bolometer measurements may be adjusted to fall within adesired range. In this regard, according to some embodiments, biascircuit 108 may be configured to set the bias based on calibration data(e.g., adjustment values stored as binary bits) stored in a calibrationdata memory 109. In other embodiments, such calibration data may beprovided from a source external to bolometer circuit 100 (e.g., from anexternal processor and/or memory), directly to bias circuit 108 or viacalibration data memory 109.

In some embodiments, bias circuit 108 may be configured to set the biasglobally for all active bolometers 102. In other embodiments, biascircuit 108 may be configured to provide a particular bias level to eachindividual one or group of active bolometers 102. In yet otherembodiments, bias circuit 108 may be configured to set a global biaslevel for all active bolometers 102 and to apply an adjustmentparticular to each one or group of active bolometers 102.

As discussed, bolometer circuit 100 includes a plurality of activebolometers 102 in an array or other arrangement. According to one ormore embodiments of bolometer circuit 100, reduction of circuitry andinterconnection may be achieved by appropriate multiplexing of activebolometers 102 to various components of the ROIC. For example, in oneembodiment, rather than replicating similar circuitry for every row 112of unit cell array 104, rows 112 may be multiplexed to column circuits114 comprising common components 116 through 128 that may be utilizedfor all rows 112 in a time-multiplexed manner As further discussedherein, components of column circuit 114 may include a load bolometer116, an amplifier 118, a feedback resistor 120, a low pass filter(“LPF”) 122, a sample-and-hold circuit 124, a comparator 126, a latch128, and/or other components, according to one or more embodiments.

In the example shown in FIG. 1, each column of unit cell array 104 has acorresponding one of column circuits 114, such that all rows of unitcells in a single column may be multiplexed to a single correspondingcolumn circuit. The plurality of column circuits 114 may in turn bemultiplexed by a column multiplexer 130, for example, to generate acombined output for unit cell array 104 in a multiplexed manner. It iscontemplated that in other embodiments, column circuits may be providedin numbers greater or fewer than the number of columns in unit cellarray 104. It is also contemplated that unit cell array 104 may compriseappropriate ROIC components to generate an output without multiplexing.It should be noted that the terms “column” and “row” herein are used asmere labels to facilitate illustration, and thus may be usedinterchangeably depending on structures being described.

In one or more embodiments, bolometer circuit 100 may include a timingand control circuit 132 configured to generate control signals formultiplexing active bolometers 102 and column circuits 114. For example,timing and control circuit 132 may be configured to control switchesassociated with active bolometers 102 and column circuits 114 toselectively connect active bolometers 102 to appropriate column circuits114 according to specified timing to enable timed-multiplexing of activebolometers 102 to column circuits 114. In some embodiments, timing andcontrol circuit 132 may be further configured to provide timed controlof other components of bolometer circuit 100. For example, portions ofbias circuit 108, blind bolometer cells, or other components may beselectively enabled and/or connected in conjunction with themultiplexing of active bolometers 102, by way of control signalsgenerated by timing and control circuit 132 according to appropriatetiming. In another example, calibration data (e.g., adjustment bitsstored in calibration data memory 109) or other data may be provided(e.g., transmitted, transferred, and/or latched) to appropriatecomponents (e.g., bias circuit 108) of bolometer circuit 100 accordingto specified timing in response to control signals from timing andcontrol circuit 132.

In some embodiments, additional switches may be provided for activebolometers 102 that allow configuration of active bolometers 102 formeasuring differences in resistance changes between the adjacent ones ofactive bolometers 102, and timing and control circuit 132 may beconfigured to generate control signals for such additional switches toenable capturing of difference images (e.g., representing differences ininfrared radiation incident on adjacent active bolometers 102) bybolometer circuit 100. As further described herein, bolometer circuit100 configured to obtain difference images according to such embodimentsmay beneficially reduce the effects on an output signal by variationsamong active bolometers 102, substrate or ambient temperature changes,and self-heating (e.g., pulse bias heating) of active bolometers 102.Further, bolometer circuit 100 according to such embodiments may beutilized to obtain images with a high scene dynamic range byreconstructing scene images from local difference of adjacent pixels.

Bolometer circuit 100 according to some embodiments may include blindbolometers 134. Blind bolometers 134 are thermally isolated (e.g.,released) from substrate 106, similar to active bolometers 102. However,unlike active bolometers 102, blind bolometers 134 are shielded frominfrared radiation from an external scene. As such, blind bolometers 134do not substantially change temperature in response to the incidentradiation level from an external scene, but do change temperature as aresult of self-heating (e.g., pulse bias heating) and temperaturechanges in substrate 106. Because both active bolometers 102 and blindbolometers 134 to a first order track temperature changes due toself-heating and substrate temperature changes, blind bolometers 134 maybe configured as references for adjusting biases for active bolometers102 and/or as references for reconstructing scene images from differenceimages according to various embodiments further discussed herein. Someexample techniques to implement blind bolometers 134 may be found inInternational Patent Application No. PCT/US2012/049051 filed Jul. 31,2012 and entitled “Determination of an Absolute Radiometric Value UsingBlocked Infrared Sensors.” Blind bolometer 134 together with itsassociated circuitry (e.g., associated switches) may be referred to as ablind bolometer cell.

Bolometer circuit 100 according to some embodiments may include a rampgenerator 136. Ramp generator 136 may be configured to generate a rampsignal for performing a ramp-compare analog-to-digital (A/D) conversionor for other use (e.g., as a reference signal in detecting a clock rate)in bolometer circuit 100. In other embodiments of bolometer circuit 100,for example in embodiments that do not include A/D conversion circuitry,ramp generator 136 may be omitted from bolometer circuit 100. Forexample, A/D conversion circuitry and a ramp generator may be externalto bolometer circuit 100 according to some embodiments.

Bolometer circuit 100 according to some embodiments may include atemperature sensor 138 configured to detect an ambient temperatureassociated with substrate 106 of bolometer circuit 100. Substratetemperature readings obtained via temperature sensor 138 may, forexample, be used to obtain and apply calibration data over a range ofsubstrate temperatures. In some embodiments, temperature sensor 138 maybe disposed on substrate 106 in close proximity to active bolometers 102and/or blind bolometers 134, so that the temperature reading obtained bytemperature sensor 138 may closely track the substrate temperatureeffecting these components. In some embodiments, bolometer circuit 100may be configured to output a substrate temperature reading based on thetemperature detected by temperature sensor 138, so that the substratetemperature reading may be accessed by components external to bolometercircuit 100 (e.g., by a processor external to bolometer circuit 100).For example, such a substrate temperature reading may be utilized toperform various correction (e.g., non-uniformity correction) andcalibration processes by a processor or other logic device.

In some embodiments, bolometer circuit 100 may include a processor orother logic device 140 configured to perform various operationsassociated with bolometer circuit 100, based on configuration datastored in a configuration data memory 142. For example, in oneembodiment, processor or other logic device 140 may be configured toperform at least part of the various processes disclosed herein below.In other embodiments, other external components (e.g., a processor of ahost device) or internal components (e.g., timing and control circuit132) may additionally or alternatively be configured to perform at leastpart of the various processes disclosed herein below.

According to one or more embodiments, processor or other logic device140 may be implemented with any appropriate combination of processingdevices, such as a general-purpose central processing unit (“CPU”), aprogrammable logic device (“PLD”) including a field programmable logicdevice (“FPGA”), a hardwired application-specific integrated circuit(“ASIC”), a digital signal processor (DSP), an image signal processor(ISP), or other logic device that may perform processing operations byexecuting instructions provided from configuration data memory 142and/or by configuring logic circuits according configuration data (e.g.,FPGA configuration data) provided from configuration data memory 142.

As discussed, FIG. 1 is a block diagram to facilitate description andexplanation of bolometer circuit 100 and its various components for oneor more embodiments of the disclosure. As such, the block diagram ofFIG. 1 is not intended to limit the size, the number, the placement, orthe orientation of the various components illustrated therein. Forexample, although blind bolometers are represented by rows of blindbolometers 134 above unit cell array 104 in FIG. 1, some or all of theblind bolometers represented by blind bolometers 134 may be provided asone or more columns of blind bolometers (e.g., to implement blindbolometers for bias columns, as further discussed herein) adjacent tocolumns of unit cell array 104 as desired for particular implementationsof bolometer circuit 100 according to various embodiments.

FIG. 2A illustrates an example circuit 200A configured to provide adesired level of bias to an active bolometer 202 to generate an output299 in accordance with an embodiment of the present disclosure. Comparedwith conventional bolometer circuits having bias control capabilities,circuit 200A according to various embodiments may advantageously achievereduction in complexity, cost, and noise while providing larger amountsof bias at a desired level to achieve an increase response to incidentinfrared radiation as further discussed herein.

Circuit 200A may represent an implementation example of a portion ofbolometer circuit 100. For example, active bolometer 202 may representone of active bolometers 102, and other components of circuit 200A mayrepresent components of a corresponding column circuit 114 and biascircuit 118, where the one of active bolometers 102 may be selectivelycoupled to the corresponding column circuit 114 (e.g., by switchingaccording to a control signal from timing and control circuit 118) toform an embodiment of circuit 200A. However, it should be recognizedthat circuit 200A is not limited to bolometer circuit 100, and thattechniques disclosed for circuit 200A are applicable to various types ofinfrared detector circuit implementations as would be understood by oneskilled in the art. For example, circuit 200A may represent a unit cellof a FPA. In another example, circuit 200A may represent a circuit toimplement a single element infrared detector.

Circuit 200A includes active bolometer 202 (labeled “R_(det)”) and abolometer 216 (also referred to as load bolometer and identified by thelabel “R_(load)”) coupled in series in a conduction path 206 (alsoreferred to as a bolometer conduction path 206) extending from a supplyvoltage node 208 to a common voltage node 210 (which a ground voltage orother common voltage level for circuit 200A). Active bolometer 202 is athermally isolated bolometer configured to receive and change itsresistance in response to incident infrared radiation 201, similar toactive bolometers 102 discussed above. Bolometer 216 provides aresistive load for conduction path 206, and is thermally shorted (e.g.,thermally shunted) to a substrate on which circuit 200A may be provided.In one example, thermally shorted bolometer 216 may be provided as acomponent (e.g., as load bolometer 116, also labeled “R_(load)” inFIG. 1) of column circuit 114 and thermally shorted to substrate 106 byapplying a thermally conductive material. Thermally shorted bolometer216 exhibits high thermal conductivity to the substrate, and thus itstemperature may be dominated by the substrate temperature, whereasincident infrared radiation and self-heating have little effect on thetemperature of thermally shorted bolometer 216. In addition, activebolometer 202 and thermally shorted bolometer 216 may be formed toexhibit the same or substantially same temperature coefficient ofresistance (TCR), for example by using the same material or differentmaterials having similar TCRs.

Because both active bolometer 202 and thermally shorted bolometer 216track varying substrate temperature to a first order, and because theymay be formed to exhibit similar TCRs, changes in substrate temperatureshould not appreciably affect the ratio of voltage drops or currentflows across active bolometer 202 and thermally shorted bolometer 216.In this regard, thermally shorted bolometer 216 may act as a substratetemperature-compensated load for circuit 200A that may cancel much ofthe effects of substrate temperature changes on a bias applied acrossactive bolometer 202. It should, however, be appreciated that othersuitable component or circuit may be used to implement a load (e.g.,including a temperature-compensated load, non-compensated load, or othervariable or non-variable load) for circuit 200A, in place of or inaddition to thermally shorted bolometer 216.

Circuit 200A includes an amplifier 218 (e.g., an operational amplifier)with one input 217 (e.g., inverting input) coupled to a node 211 betweenactive bolometer 202 and thermally shorted bolometer 216 in conductionpath 206. An output 219 of amplifier 218 may be coupled to its input 217by another thermally shorted bolometer 220 (labeled “R_(f)”) that actsas a feedback resistor, thereby configuring amplifier 218 as a feedbackamplifier (e.g., a transimpedance amplifier). As with thermally shortedbolometer 216, the temperature (and therefore the resistance) ofthermally shorted bolometer 220 tracks the substrate temperature, butmay not be appreciably affected by incident infrared radiation 201 orself-heating. Thus, thermally shorted bolometer 220 may act as asubstrate temperature-compensated feedback resistor that substantiallymaintains a desired resistance relative to active bolometer 202 andthermally shorted bolometer 216, and therefore maintains a desired gainratio for the feedback amplifier, over a range of substrate temperature.Amplifier 218, thermally shorted bolometer 220, and other componentsthat may be associated with amplifier 218 and/or thermally shortedbolometer 220 may be referred to herein as belonging to an amplifiercircuit for circuit 200A.

Another input 221 (e.g., non-inverting input) of amplifier 218 may beprovided with a reference voltage V_(ref). Due to high impedance betweeninputs 221 and 217 of amplifier 218 and the feedback configuration, thevoltage potential at input 217 of amplifier 218 follows V_(ref), thevoltage provided at input 221. In other words, V_(ref) provided at input221 may set a virtual ground for the feedback amplifier configuration,which is also the voltage potential at input 217. Because input 217 iscoupled to node 211, active bolometer 202 is biased by V_(ref) at node211 and by the voltage at common voltage node 210, while thermallyshorted bolometer 216 is biased by the supply voltage at supply voltagenode 208 and by V_(ref) at node 211. That is, for example, V_(ref)provided to amplifier 218 can be used to set biases across both activebolometer 202 (e.g., detector bolometer R_(det)) and thermally shortedbolometer 216 (e.g., load bolometer R_(load)) virtually, without a needfor extra circuitry (e.g., CMOS transistors) in conduction path 206 toset and maintain desired bias levels.

In various embodiments, biases across active bolometer 202 and thermallyshorted bolometer 216 may be set according to V_(ref) such that theoutput voltage at amplifier 218 is in a desired range in response to theresistance changes of active bolometer 202 due to incident infraredradiation 201. For example, in some embodiments, V_(ref) may be set suchthat a current 203 (labeled “I_(load)” and also referred to as a loadcurrent), which is generated through the load bolometer R_(load) (e.g.,thermally shorted bolometer 216), in response to the voltage potentialbetween the supply voltage level at supply voltage node 208 and thevoltage level V_(ref) at node 211 applied across the load bolometerR_(load), is to a first order the same or substantially same as acurrent 205 (labeled “I_(det)”) through the detector bolometer R_(det)(e.g., active bolometer 202) less a current 207 (labeled “I_(scene)”)due to the resistance changes in response to incident infrared radiation201 from an external scene. Thus, in such embodiments, the differencebetween current 205 (I_(det)) through active bolometer 202 and current203 (I_(load)) through thermally shorted bolometer 216 may represent thecurrent 207 (I_(scene)) that is due to the resistance change of activebolometer 202 in response to incident infrared radiation 201.

This current 207 (I_(scene)) due to incident infrared radiation 201flows through thermally shorted bolometer 220 (feedback bolometer R_(f))since thermally shorted bolometer 220 represents a low-impedance pathfor amplifier 218 in the feedback configuration. The voltage (V_(out))at output 219 of amplifier 218 is thenV_(out)=I_(scene)×resistance(R_(f)), where resistance(R_(f)) representsthe resistance value of thermally shorted bolometer 220. Thus, forexample, the scene current to output voltage gain is approximately theratio of resistance(R_(f))/resistance(R_(det)). Advantageously, thisgain ratio may remain substantially constant over a varying substratetemperature, since the temperature, and therefore the resistance change,of both thermally shorted bolometer 220 and active bolometer 202 trackthe substrate temperature to a first order as discussed above. It isalso contemplated for other embodiments that other variable ornon-variable impedance component or circuit may be utilized to implementa feedback resistor for amplifier 218, in addition to or in place ofthermally shorted bolometer 220.

In various embodiments, the reference voltage V_(ref) at input 221 maybe supplied by a variable voltage source 250 coupled to input 221. Forexample, in some embodiments, variable voltage source 250 may beimplemented using a digital-to-analog converter (DAC), such as a CMOSDAC or other suitable DAC that can be used to output a desired voltagelevel by providing corresponding digital data (e.g., binary bits). Othersuitable variable voltage sources that allow their output voltage to beadjusted (e.g., by appropriate inputs or control signals) may be used inother embodiments to implement variable voltage source 250. Thus, byproviding appropriate binary bits to a CMOS DAC implementing variablevoltage source 250 or otherwise adjusting the output voltage of variablevoltage source 250 to set the reference voltage V_(ref), the biasesacross active bolometer 202 and thermally shorted bolometer 216 may beset as desired, for example, such that the current that flows throughthermally shorted bolometer 220 to generate the output voltage V_(out)is predominantly or exclusively current 207 (I_(scene)) associated withthe resistance change at active bolometer 202 due to incident infraredradiation 201 as illustrated above.

As may be appreciated, natural variations in performance characteristicsof various components may exist when a device implementing circuit 200Ais fabricated. For example, small deviations from the intended designparameters may exist in the infrared radiation absorption coefficient,resistance, TCR, heat capacity, and thermal conductivity associated withbolometers 202, 216, and 220 when fabricated. According to variousembodiments of circuit 200A, biases for active bolometer 202 andthermally shorted bolometer 216 may be adjusted to correct for suchdeviations by conveniently adjusting the reference voltage V_(ref) viavariable voltage source 250.

If, for example, circuit 200A is implemented in a FPA, such as in anembodiment of bolometer circuit 100, such variations resulting fromfabrication processes may result in non-uniformity of responses amongpixels in an FPA. As discussed above for one example, one of activebolometers 102 in an array may be selectively coupled to a correspondingone of column circuits 114, each of which comprises load bolometer 116,feedback resistor 120, and amplifier 118, to form circuit 200A, wherethe selected active bolometer 102, load bolometer 116, feedback resistor120, and amplifier 118 may respectively correspond to bolometers 202,216, 220, and amplifier 218 of circuit 200A. In such an embodiment ofbolometer circuit 100, different combinations of active bolometers 102and column circuits 114 may exhibit different response characteristics(e.g., producing different output voltages for a given incident infraredradiation), if the reference voltage V_(ref), and therefore the biasesfor active bolometers 102 and load bolometers 116, are not adjusted foreach combination to correct for fabrication variations or othervariations.

In that regard, according to some embodiments, offset adjustmentcircuitry 260 may be provided at one or more various locationsdesignated with labels A, B, C, and D on circuit 200A to enableadditional and/or fine adjustment to the bias. For example, in someembodiments, offset adjustment circuitry 260 may comprise a variablevoltage source (e.g., a DAC) at locations A, B, or both, such that thebias across active bolometer 202, thermally shorted bolometer 216, orboth may be further adjusted by varying the voltage level at locationsA, B, or both. Thus, for example, if circuit 200A is implemented inbolometer circuit 100, biases across active bolometers 102, loadbolometers 116, or both may be further adjusted on a per-pixel basis(e.g., providing adjustment specific to each pixel) to correct fornon-uniformities using offset adjustment circuitry 260 comprising one ormore variable voltage sources.

In some embodiments, offset adjustment circuitry 260 may be provided atlocation C and configured to modify or otherwise adjust the voltagesupplied by variable voltage source 250 to input 221 of amplifier 218 toprovide per-pixel adjustment of the reference voltage V_(ref), therebyproviding per-pixel adjustment of the biases. In some embodiments,offset adjustment circuitry 260 may be provided as part of or as anaddition to amplifier 218 (e.g., at location D) and configured to alterone or more characteristics of amplifier 218 in response to receivingcalibration data (e.g., adjustment bits) for each pixel to provideper-pixel adjustment of the biases. Embodiments that provide offsetadjustment at locations C and/or D beneficially allow offset adjustmentcircuitry 260 to be removed from conduction path 206. Because offsetadjustment circuitry 260 may be removed from conduction path 206 inthese embodiments, a larger portion of the supply voltage mayadvantageously be available for biasing active bolometer 202 inconduction path 206, and offset adjustment circuitry 260 may not besubject to, and thus need not be designed to operate under, largevariations in the current flowing through conduction path 206.

It is also contemplated for some embodiments that variable voltagesource 250 may be utilized, in addition to or in place of offsetadjustment circuitry 260, to provides specific adjustment for each pixelor a group of pixels in a FPA, instead of providing a uniform referencevoltage V_(ref) for an entire FPA to set a global bias.

Circuit 200A may include a low pass filter (“LPF”) 222 coupled to output219 of amplifier 218, according to some embodiments. As discussed above,the output voltage (V_(out)) at output 219 may represent a signalindicative of an intensity of incident infrared radiation 201 to bedetected. LPF 222 coupled to output 219 may then limit a noise bandwidthin such a signal by filtering out high frequency noise components. Invarious embodiments, LPF 222 may be implemented as a resistor-capacitor(RC) network LPF, switched capacitor LPF, or any other suitable LPFimplementation. In some embodiments, circuit 200A may include asample-and-hold circuit 224 configured to receive the output voltage(V_(out)) from amplifier 218 or the filtered signal from LPF 222 andhold it (e.g., substantially maintain a same voltage level) for apredetermined time before passing it as an analog output signal (e.g.,output 299) for circuit 200A. The analog output signal may then befurther processed (e.g., converted into digital signals) or otherwiseutilized as desired for application of circuit 200A.

Therefore, various embodiments of circuit 200A described above may beconfigured to set and maintain a desired level of bias to activebolometer 202 and thermally shorted bolometer 216 to generate anamplified output signal indicative of an intensity of incident infraredradiation 201 received at active bolometer 202. Whereas conventionalbolometer circuits typically required a significant amount of circuitryfor bias control, various embodiments of circuit 200A may advantageouslyachieve bias generation and control without much of the conventionalbias control circuitry and the accompanying complexity, size, cost, andnoise.

Specifically, for example, various circuits and components such as CMOStransistors required for conventional bias control may be removed fromconduction path 206 of active bolometer 202 and thermally shortedbolometer 216 according to various embodiments of circuit 200A. Inaddition, associated reference bolometers and other reference circuitryrequired to generate reference signals (e.g., to drive the gates of CMOStransistors on conduction paths 206) for conventional bias circuitry maynot be required for various embodiments of circuit 200A.

Removing components such as bias-controlling CMOS transistors fromconduction path 206 not only reduces complexity, size, and costassociated with such circuitry, but also removes noise that may beintroduced by CMOS transistors and other circuitry from a sensitivefront-end (e.g., conduction path 206) of circuit 200A. In addition, byusing a low impedance variable voltage source 250 such as a CMOS DAC toset biases, circuit 200A according to various embodiments maybeneficially exhibit lower noise compared with conventional bolometercircuits with bias control capabilities. Furthermore, by removingconventional circuitry such as reference circuitry and bias-controllingCMOS transistors that limit current flow and/or voltage to control bias,a much larger portion of a supply voltage (e.g., a voltage potentialbetween the supply voltage at supply voltage node 208 and the common orground voltage at common voltage node 210) may be dedicated to biasingactive bolometer 202. Consequently, for example, a larger bias may beprovided to active bolometers 202 for an improved response.

Further in this regard, various embodiments of circuit 200A may providea desired level of output via amplifier 218 in a feedback configurationusing thermally shorted bolometer 220, without a need for an integratingamplifier (e.g., using a large capacitor) to provide a large gain astypically required in conventional bolometer circuits (e.g., requireddue to a smaller bias). Amplifier 218 in a feedback configuration usingthermally shorted bolometer 220 may not only have a size advantage overan integrating amplifier that typically requires a large capacitor andadditional buffers, but may also permit more convenient and stablecontrol over a gain ratio. That is, a desired gain ratio mayconveniently be set for circuit 200A by selecting an appropriateresistance value for thermally shorted bolometer 220, which may also actas a substrate-temperature compensated feedback resistor to provide astable gain ratio over a varying substrate temperature as discussedabove. In some embodiments, for example, one or more switches 223 (e.g.,CMOS switches) may be provided that may each be turned on or off toselectively short corresponding one or more portions of thermallyshorted bolometer 220, thereby adjusting the resistance value forthermally shorted bolometer 220 and in turn adjusting the gain ratio forcircuit 200A. Thus, in such embodiments, appropriate ones of the one ormore switches 223 may be turned on or off (e.g., by providing controlbits or control signals to switches 223) to conveniently adjust the gainratio for circuit 200A, for example, in case there is a large amount ofsignal such as when incident IR radiation 201 comes from a fire or a hotobject. In contrast, to control the gain ratio for an integratingamplifier in conventional bolometer circuits, a complicated process ofadjusting the integration time may be required.

These beneficial aspects of circuit 200A according to variousembodiments may thus permit infrared sensors, FPAs for infrared imaging,or other sensor or imaging devices to be realized with lower cost,complexity, noise, and/or a smaller footprint than achievable withconventional bolometer circuits. Such reduction in complexity, size,noise, and cost may beneficially lead to infrared imaging FPAs or otherimaging or sensor devices with a higher resolution (e.g., a largernumber of pixels at a smaller pixel pitch) and performance (e.g., betterresponse and low noise), yet having a lower manufacturing cost, asmaller size and a lower power requirement, for example.

Circuit 200A of FIG. 2A discussed above is an exemplary circuit toillustrate various techniques to provide a desired level of bias todetector bolometers to generate an output signal indicative of anintensity of incident infrared radiation according to variousembodiments of the present disclosure. As such, it should be understoodthat various techniques discussed above for circuit 200A may beimplemented in a wide variety of circuit devices, and that numerousmodifications and variations are possible in accordance with theprinciples of the present disclosure. For example, circuit polaritiesmay be inverted, such as by inverting power supplies and invertingpolarities for relevant circuit components. Furthermore, various paths,nodes, and locations discussed above in connection with circuit 200A areidentified for purposes of illustration, and should not be understood aslimited to specific physical locations of circuit 200A. Rather, oneskilled in the art may recognize corresponding paths, nodes, andlocations in a circuit or device implementing the principles of thedisclosure illustrated in connection with circuit 200A. In addition, thedirection of current flow in connection with circuit 200A are identifiedfor purposes of illustration, and one skilled in the art shouldrecognize that the illustrated directions may be reversed or notdepending on actual voltage potentials during operation of circuit 200A.

FIG. 2B illustrates an example of a circuit 200B similar to circuit200A, but further configured to enable a low-power mode of operation inaccordance with an embodiment of the disclosure. For example, circuit200B may be selectively operated in a low-power mode or a normal mode.In the low-power mode according to some embodiments, amplifier 218and/or conduction path 206 are disconnected from power while input 217and output 219 of amplifier 218 are driven to a predetermined voltagelevel. In the normal mode, circuit 200B operates in the manner describedabove for circuit 200A to generate an output signal indicative of anintensity of incident infrared radiation 201 from an external scene atoutput 219 of amplifier 218.

In this regard, circuit 200B may include a plurality of switches 240-244(also labeled “SW1” through “SW5”) associated with load bolometer 216,active bolometer 202, and amplifier 218, according to one or moreembodiments of the disclosure. In one or more implementation, switches240-244 (SW1-SW5) may be implemented as CMOS switches. In theillustrated embodiment, switch 240 (SW1) is configured to selectivelyconnect or disconnect load bolometer 216 to or from supply voltage node208, while switch 241 (SW2) is configured to selectively connect ordisconnect active bolometer 202 to or from common voltage node 210.Thus, by selective opening and closing of switches 240 and 241,bolometer conduction path 206 can be connected to or disconnected frompower. In the illustrated embodiment, switch 244 (SW5) is configured toselectively connect or disconnect amplifier 216 to or from commonvoltage node 210, thereby selectively providing or cutting power toamplifier 216. As discussed above for circuit 200A, it should beappreciated that the polarities for relevant circuit components ofcircuit 200B may be reversed depending on specific implementation aswould be understood by one skilled in the art. Accordingly, in otherembodiments, switch 240 may be provided to selectively connect ordisconnect associated load bolometer 216 to or from common voltage node210, and switches 241 and 245 may be provided to selectively connect ordisconnect the associated components to or from supply voltage node 208,for example.

In the illustrated embodiment, switch 242 (SW3) is configured toselectively connect or disconnect output 219 of amplifier 218 to or frominput 217 of amplifier (e.g., selectively short output 219 of amplifier218 to its input 217), and switch 243 (SW4) is configured to selectivelyconnect or disconnect output 219 of amplifier to supply voltage node208. In other embodiments, switch 243 (SW4) may be configured toselectively connect or disconnect output 219 of amplifier to a node thatsupplies a predetermined voltage level other than the supply voltagelevel at supply voltage node 208. Thus, for example, output 219 ofamplifier 218 may be driven to and held a stable level (e.g., the supplyvoltage level at supply voltage node 208) by closing switch 243 (SW4).Furthermore, for example, both inputs 217 and 221 and output 219 ofamplifier 218 maybe be driven to and held at a stable level (e.g., asupply voltage level at supply voltage node 208) by opening switches 240and 241 (SW1 and SW2) to disconnect conduction path 206 from power andby closing switches 243 and 244 (SW3 and SW4) to connect output 219 ofamplifier 218 to supply voltage node 208 and short output 219 to input217 of amplifier 218. With inputs 217 and 221 and output 219 ofamplifier 218 held at a stable level, power can be cut from amplifier218 by opening switch 240 (SW5) as discussed above to reduce powerconsumption by circuit 200B while providing a stable, predeterminedlevel of output.

Therefore, in the illustrated embodiment, circuit 200B may be operatedin a normal mode by setting switches 240, 241, and 244 (SW1, SW2, andSW5) in a closed position and switches 242 and 243 (SW3 and SW4) in anopen position, and operated in a low-power mode by setting switches 240,241, and 244 (SW1, SW2, and SW5) in an open position and switches 242and 243 (SW3 and SW4) in a closed position. In another embodiment,circuit 200B may be operated in a low-power mode by setting switch 240(SW1) in an open position and omitting switch 241 (e.g., the electricalpath is not broken), or by setting switch 241 (SW2) in an open positionand omitting switch 240 (e.g., the electrical path is not broken),thereby disconnecting bolometer conduction path 206 from power while atthe same time driving node 211 and input 217 of amplifier 218 to apredetermine voltage level (e.g., based on the supply voltage level ifswitch 240 is omitted or on the common voltage level if switch 241 isomitted). Amplifier 218 is also disconnected from power and its outputis driven to a predetermined level (e.g., the supply voltage level) bysetting switch 243 in a closed position and switch 244 in an openposition. Since node 211 and input 217 is driven to a predeterminevoltage level through part of bolometer conduction path rather than byoutput 219 of amplifier 218, switch 242 (SW3) may also be omitted inthis embodiment. In yet another embodiments, circuit 200B may beoperated in a low-power mode by setting switches 240, 241 and 243 (SW1,SW2 and SW4) in an open position and switches 242 and 244 (SW3 and SW5)in a closed position. Such a low-power mode may also referred to as akilled-pixel mode and provide less power saving, but may allow acontrolled response and current levels for a bad (opened or shorted)active bolometer while continuing to drive the reference voltage levelonto node 211, for example, in preparation for the next active bolometerin the column. As may be appreciated, not all of switches 240-244 arerequired in those embodiments with a killed-pixel or other low-powermodes. In some embodiments, a control circuit, such as for exampletiming and control circuit 132 and/or processor 140, may be configuredto generate control signals to set switches 240-244 to operate circuit200B in a low-power mode or a normal mode.

As discussed above, when operated in a low-power mode, bolometerconduction path 206 and amplifier 218 are disconnected from power toreduce power consumption of circuit 200B, while at the same timeproviding a stable, predetermined level of output for circuit 200B. As aresult, when circuit 200B is implemented as part of a larger circuit ordevice, circuit 200B can be selectively operated in the low-power modeto save power without undesirably affecting the functionality of otherparts of the larger circuit or device. For example, a plurality ofcircuits 200B may be implemented in a FPA, such as in an embodiment ofbolometer circuit 100, which may include a circuit 200B for each columnor unit cell. In such implementations, circuits 200B for some columns orunit cells may be operated in the normal mode while the remainder ofcolumns or unit cells may be operated in the low-power mode, therebyreducing the overall power consumption by the FPA. Furthermore, theoutputs of those columns or unit cells that are operated in thelow-power mode will not appear as noise or random artifacts but willrather be held at a stable, predetermined value as discussed above,which may beneficially permit IR images captured by the FPA to be usedfor some purposes even when some columns or unit cells are in thelow-power mode. Further examples of an FPA configured to use part of theFPA in the low-power mode is discussed below with reference to FIG. 4Din accordance with one or more embodiments of the disclosure. Althoughcircuit 200B is illustrated in FIG. 2B as having circuit 200A as a baseconfiguration, it should be appreciated that the techniques andprinciples described in connection with embodiments of circuit 200B arealso applicable to other suitable bolometer circuits having an activebolometer, a resistive load, an amplifier, and/or other componentsgenerally found in bolometer circuits.

FIG. 3 illustrates a bolometer circuit 300 comprising an embodiment ofcircuit 200A in accordance with an embodiment of the disclosure.Bolometer circuit 300 may include one or more column circuit blocks 314(individually identified as column circuit block 314(1) through 314(M),where M may represent the desired number of columns in unit cell array104). Each column circuit block 314 may be associated with one or moreactive bolometers 302. In FIG. 3, only those one or more activebolometers 302 (individually identified as column circuit block 302(1)through 302(N), where N may represent the desired number of rows in unitcell array 104) that are associated with column circuit block 314(1) areshown for clarity, but it will be understood that other activebolometers 302 not explicitly shown in FIG. 3 may be provided andassociated with other column circuit blocks 314(2) through 314(M).

Each active bolometer 302 may be accompanied by switches 370 and 372 forselectively enabling and connecting to a corresponding one of columncircuit blocks 314. In some embodiments, one or more blind bolometers(not shown in FIG. 3) may additionally be associated with each columncircuit block 314. Such blind bolometers may be implemented in a same orsimilar manner as blind bolometer 134 discussed above for bolometercircuit 100 and configured to be selectively enabled and connected tothe each column circuit block 314 via accompanying switches, forexample, to provide a reference signal level for calibration and otherpurposes.

In some aspects, column circuit block 314 may represent one of columncircuits 114, and active bolometer 302 may represent one of activebolometers 102 of bolometer circuit 100, where active bolometer 102 maybe selectively coupled to a corresponding one of column circuits 114 viaswitches to form a circuit comprising an embodiment of circuit 200A asdiscussed above. In this regard, bolometer circuit 300 according tovarious embodiments may further include a column multiplexer 330, acalibration data memory 309, a timing and control circuit 332, a rampgenerator 336, a temperature sensor 338, a processor or other logicdevice 340, and/or a configuration data memory 342, all of which may beimplemented in a similar manner as their corresponding components ofbolometer circuit 100.

According to various embodiments, each column circuit block 314 mayinclude a thermally shorted bolometer 316, an amplifier 318 (anoperational amplifier), a thermally shorted bolometer 320 (e.g.,feedback bolometer), a LPF 322 (e.g., a RC LPF), and/or asample-and-hold circuit 324, which may respectively correspond tothermally shorted bolometer 216, amplifier 218, thermally shortedbolometer 220, LPF 222, and sample-and-hold circuit 224 of circuit 200A.Amplifier 318, thermally shorted bolometer 320, and other componentsthat may be associated with amplifier 318 and/or thermally shortedbolometer 220 may be referred to herein as belonging to an amplifiercircuit for column circuit block 314.

As may be appreciated, various components of column circuit block 314may be configured to form corresponding portions of circuit 200Adiscussed above. For example, as with circuit 200A, thermally shortedbolometer 320 may be coupled in parallel across an inverting input 317and an output 319 of amplifier 318, thereby configuring amplifier 318 asa feedback amplifier. Thermally shorted bolometer 320 may be configuredwith a resistance to provide a desired gain. For example, thermallyshorted bolometer 320 may be configured to exhibit a resistance Rf×Rb asshown in FIG. 3, where Rf may represent a gain factor and Rb mayrepresent a resistance of active bolometer 302.

In various embodiments, thermally shorted bolometer 316 may be coupledto supply voltage node 308 at one end, and to a circuit path 306 leadingto one or more associated active bolometers 302(1)-302(N) at the otherend. Active bolometers 302(1)-302(N) may be implemented in a same orsimilar manner as active bolometer 202 of circuit 200A. In general, theresistance of thermally shorted bolometer 316 and the resistance ofactive bolometer 302 may be determined as desired for particularimplementations of bolometer circuit 300, taking into account, forexample, a supply voltage range, operating characteristics of othercomponents of bolometer circuit 300, a desired range of bias, and otherimplementation parameters. For example, the ratio of the resistance ofthe resistance of thermally shorted bolometer 316 and the resistance ofactive bolometer 302 may be set to K, such as by providing a thermallyshorted bolometer 316 having a resistance of K×Rb, where K may be anydesired value for particular implementation parameters and need not bean integral number. In the illustrated embodiment, thermally shortedbolometers 316 and active bolometers 302 have a similar resistance(i.e., the resistance ratio of thermally shorted bolometers 316 toactive bolometer is approximately 1).

In various embodiments, one of active bolometers 302(1)-302(N) may becoupled to thermally shorted bolometer 316 via switch 370 at one end,and to an offset-adjustment DAC (ODAC) 360 via another switch 372 at theother end. For example, one or more active bolometers 302 may each beselectively enabled and electrically connected to thermally shortedbolometer 316 and ODAC 360 of a corresponding one of column circuitblocks 314 (e.g., via switches 370 and 372 being open or closedaccording to control signals from timing and control circuit 332) toform a bolometer conduction path and operate in a same or similar manneras active bolometer 202 discussed above for circuit 200A. Switches 370and 372 and associated switching circuitry (e.g., implemented as part oftiming and control circuit 332) may be implemented using appropriatetechniques for multi-pixels bolometer circuits, including suchtechniques, for example, as those described in U.S. Pat. No. 6,812,465entitled “Microbolometer Focal Plane Array Methods and Circuitry” andU.S. Pat. No. 7,679,048 entitled “Systems and Methods for SelectingMicrobolometers within Microbolometer Focal Plane Arrays,” which areincorporated herein by their entireties.

Thus, in one aspect, biases across a connected and enabled one of activebolometers 302 and thermally shorted bolometer 316 may be set andmaintained in a manner discussed above for circuit 200A. In this regard,inverting input 317 of amplifier 318 may be coupled to a node(designated “Col”) between thermally shorted bolometer 316 and activebolometers 302 on circuit path 306, whereas non-inverting input 321 ofamplifier 318 may be provided with a reference voltage V_(ref).Configured as such, a connected and enabled one of active bolometers 302may be biased by an output 362 of ODAC 360 and by the reference voltagelevel V_(ref) appearing at node Col as discussed above for circuit 200A.

In various embodiments, ODAC 360 of each column circuit block 314 may becoupled to supply voltage node 308 and common voltage node 310, andconfigured to generate a desired voltage level at its output 362 inresponse to offset adjustment bits 364 (e.g., a string of binary bitsindicative of a desired offset voltage). As such, ODAC 360 according tosome embodiments may implement offset adjustment circuitry 260 forbolometer circuit 300. That is, ODAC 360 of each column circuit block314 may apply an adjustment to the bias across the active bolometer 302that is selectively enabled and electrically connected to each columncircuit block 314.

In this regard, in some embodiments, ODAC 360 may be used to adjust theactive bolometer bias for the corresponding column circuit block 302. Insome embodiments, ODAC 360 may be used to apply a per-pixel adjustmentof the bias across active bolometers 302. In such embodiments,calibration data memory 309 may be configured to store correspondingoffset adjustment bits 364 for every pixel (e.g., for each one of activebolometers 302) in bolometer circuit 300, and timing and control circuit332 may be configured to supply corresponding offset adjustment bits 364to ODAC 360 of a corresponding one of column circuit blocks 314 inconnection with the switching and enabling of each row of activebolometers 302. In various embodiments, ODAC 360 may comprise aresistor-network DAC, a CMOS DAC, or any other suitable type of DAC forimplementing offset adjustment circuitry 260.

In various embodiments, bolometer circuit 300 may include a CMOS biascircuit 350 coupled to non-inverting input 321 of amplifier 318 toprovide the reference voltage V_(ref). In this regard, CMOS bias circuit350 according to one or more embodiments may comprise a CMOS DACconfigured to generate a desired voltage level in response to biasadjustment bits 352 (e.g., a string of binary bits indicative of adesired reference voltage V_(ref)). Consequently, CMOS bias circuit 350according to various embodiments may implement variable voltage source250 for bolometer circuit 300. That is, in bolometer circuit 300, CMOSbias circuit 350 may be configured to provide the reference voltageV_(ref) at a desired level based on bias adjustment bits 352 (e.g.,stored in and supplied from calibration data memory 309) for setting andmaintaining biases globally for all column circuit blocks 314 and allrows of active bolometers 302. In some embodiments, CMOS bias circuit350 may also comprise associated circuitry to latch and/or load biasadjustment bits 352, and may correspond to bias circuit 108 of bolometercircuit 100.

With a desired level of bias applied, a resistance change due toincident infrared radiation at active bolometers 302 produces anamplified output voltage at output 319 of amplifier 318, as discussedabove for circuit 200A. Output 319 of amplifier 318 is coupled to LPF322, which in embodiments of bolometer circuit 300 is implemented as aresistor-capacitor (RC) network LPF (as illustrated for example in FIG.3). At the other end, LPF 322 is coupled to sample-and-hold circuit 324,which may be implemented using one or more switches and one or morecapacitors (as illustrated for example in FIG. 3) to substantiallymaintain filtered analog voltage level (e.g., filtered analog signal)indicative of an intensity of incident infrared radiation received atactive bolometers 302.

In various embodiments, each column circuit block 314 may include acomparator 326, switches 327, capacitors 329, and latches 328, which maybe utilized to convert the filtered analog voltage level captured atsample-and-hold circuit 324 into a digital output value (e.g., byperforming a ramp-compare A/D conversion). For example, comparator 326may be configured to receive the voltage level from sample-and-holdcircuit 324 and the ramp signal from ramp generator 336, and to comparethe voltage level and the ramp signal to trigger (e.g., generate asignal to close switches 327) when the ramp signal substantially matchesthe voltage level.

In this regard, bolometer circuit 300 may also include a counter 331(e.g., a digital counter in some embodiments) configured to increment(or decrement depending on the implementation of counter 331) a countvalue (e.g., encoded in one or more count signals) in response toreceiving a clock signal. The count value incremented or decremented bycounter 331 may have a substantially similar period (e.g., resets tozero or a base value at substantially the same time) as the ramp signalgenerated by ramp generator 336. When comparator 326 triggers, thecurrent count value may be selected and stored in latches 328 as adigital value. The output of latches 328 of each column circuit block314 may be coupled to column multiplexer 330 configured to multiplex thedigital values stored in latches 328 for each column circuit block 314to generate a digital output signal 390 for all columns in bolometercircuit 300. In various embodiments, timing and control circuit 332,processor or other logic device 340, and/or other component of bolometercircuit 300 may be configured to repeat generation of digital outputsignal 390 for all rows of active bolometers 302 according topredetermined timing, such that a concatenation of digital output signal390 may digitally represent an image frame of infrared radiationreceived at active bolometers 302 of bolometer circuit 300. Thus, forexample, embodiments of bolometer circuit 300 may configured to capturea sequence of infrared image frames.

Therefore, various embodiments of bolometer circuit 300 discussed withrespect to FIG. 3 may implement one or more embodiments of circuit 200Afor multiple columns and rows of active bolometers 302 that form a FPAto generate analog and/or digital output of infrared imaging data. Otherbolometer circuits that implement one or more embodiments of circuit200A in the context of a FPA in accordance with other embodiments of thepresent disclosure are illustrated below with respect to FIGS. 4Athrough 8.

FIG. 4A illustrates a bolometer circuit 400A comprising an embodiment ofcircuit 200A in accordance with another embodiment of the disclosure. Inone aspect, for example, implementations of variable voltage source 250for various embodiments of bolometer circuit 400A may advantageouslyvary the reference voltage V_(ref) to compensate for self-heating ofactive bolometers.

Self-heating of active bolometers generally occurs due to the currentflow (e.g., I_(det) illustrated in FIG. 2) and resulting powerdissipation through active bolometers while they are biased to obtaineda measurement of incident infrared radiation. In that sense,self-heating of active bolometers may also be referred to as biasheating or pulse bias heating, for example in cases where activebolometers of a FPA are periodically connected to a column circuit andbiased for some duration as discussed above for FIGS. 1 and 3, whichresults in pulses of active bolometer heating and cooling. As brieflydiscussed above, the temperature, and therefore the resistance, ofactive bolometers is affected by self-heating since active bolometersare isolated (e.g., released) from a substrate, whereas the temperatureof thermally shorted bolometers are not appreciably affected byself-heating because of the thermal shorting to the substrate acting asa heat sink.

As may be appreciated, temperature changes of an active bolometer due toself-heating may limit the usable output signal range (or output signalswing) for measuring incident infrared radiation received at the activebolometer, even when a thermally shorted bolometer is used as atemperature-compensated load, and even when biases for the activebolometer and the thermally shorted bolometer are set and maintained ata predetermined level. To reduce such undesirable effects of activebolometer self-heating, various embodiments of bolometer circuit 400Amay vary the bias-setting reference voltage V_(ref) in response toself-heating of active bolometers to correct for the resistance changesof active bolometers due to self-heating.

More specifically, in various embodiments, a bias circuit 450implementing variable voltage source 250 for bolometer circuit 400A maycomprise one or more bias columns 454 each providing (e.g., at a nodelabeled “BCol” in FIG. 4A) a voltage level that varies in response toself-heating of one or more associated blind bolometers 434 that trackself-heating of active bolometers 302. For example, in FIG. 4A, biascolumns 454(1) through 454(B) are individually identified where “B”denotes the desired number of bias columns in bias circuit 450, andblind bolometers 434(1) through 434(R₁) are individually identifiedwhere “R₁” denotes the number of blind bolometers associated with biascolumn 454(1). Other bias columns 454(2) through 454(B) may each beassociated with a corresponding number (denoted R₂, R₃, . . . , R_(B))of blind bolometers (not explicitly shown in FIG. 4A), where R₁, R₂, R₃,. . . , R_(B) may be same or different depending on embodiments.

Blind bolometers 434 may be implemented in a similar manner as blindbolometers 134 discussed above in connection with FIG. 1. That is, blindbolometers 434 are thermally isolated (e.g., released) from a substratewhile being substantially shielded (e.g., shielded to the extent allowedby a typical fabrication process) from incident infrared radiation, andthus their temperatures change due to self-heating and substratetemperature changes but not incident infrared radiation. In one or moreembodiments, blind bolometers 434 may be implemented to exhibit asimilar TCR and resistance value as active bolometers 302.

Each bias column 454 comprises a thermally shorted bolometer 456 thatmay be implemented in a similar manner as thermally shorted bolometer316 and act as a temperature-compensated load. Thermally shortedbolometer 456 may be selectively coupled to one of blind bolometers 434in series in a conduction path extending from supply voltage node 308 tocommon voltage node 310, thereby mirroring the conduction pathcomprising thermally shorted bolometer 316 and active bolometer 302 formeasuring incident IR radiation. Because of such mirroring of activebolometers 302 using blind bolometers 434 that track resistance changesat active bolometers 302 due to self-heating, each bias column 454 mayact as a voltage divider that provides a varying voltage level (e.g.,when taken from the node labeled BCol) indicative of at what level thebias-setting reference voltage V_(ref) should be in order for the outputvoltage (e.g., output signal) of an amplifier 418 of column circuitblock 414 to represent predominantly or exclusively incident infraredradiation from a scene given the temperature variations attributable toself-heating at active bolometer 302. In this sense, bias columns 454may also be referred to as reference conduction paths.

In this regard, node BCol between thermally shorted bolometer 456 and aconnected and enabled one of blind bolometers 434 may be coupled to aninput 421 (e.g., a non-inverting input) of amplifier 418 to supply avoltage level at node BCol as the reference voltage V_(ref) to amplifier418. In one or more embodiments, node BCol of each bias column 454 andinput 421 of amplifier 418 may be coupled via a buffer 458 configured toadjust an input voltage from BCol in response to bias adjustment bits452 and to output the adjusted input voltage. In such embodiments, biasadjustment bits 452 (e.g., stored in calibration data memory 309) may beapplied to make a global fine adjustment to the varying,self-heating-compensating voltage level provided by one or more biascolumns 454, for example. If two or more bias columns 454 are provided(e.g., B>=2), the voltage levels at nodes BCol of bias columns 454(1)through 454(B) are averaged by virtue of parallel paths respectivelyconnecting nodes BCol of columns 454(1) through 454(B) to buffer 458,thereby providing a more accurate reference voltage V_(ref) to amplifier418, for example. In this regard, some embodiments may include a columnswitch 453 for each bias column 454, which may be opened or closed(e.g., by a control bit) to selectively disconnect a corresponding biascolumn 454, in case a particular bias column 454 is not providing anaccurate reference voltage V_(ref), for example.

As discussed above, each bias column 454 may comprise one or moreassociated blind bolometers 434. In some embodiments, a plurality ofassociated blind bolometers 434 may be provided for each bias column 454and selectively enabled and connected to thermally shorted bolometer 456to form the voltage divider discussed above. More specifically for someembodiments, switches 433 and 435 may be provided for each blindbolometer 434, and selectively closed or opened according to appropriatetiming based on signals from timing and control circuit 332. Forexample, timing and control circuit 332 may be configured to controlswitches 433 and 435 for blind bolometers 434 and switches 370 and 372for active bolometers 302, such that each blind bolometer 434 may beselectively enabled and connected to produce a voltage level at nodeBCol in connection with (e.g., synchronous with) the selection andenabling of a corresponding one of active bolometers 302, therebyallowing the enabled and connected one of blind bolometers 434 to trackself-heating of the corresponding one of active bolometers 302 whilebeing biased (e.g., during its bias period or bias pulse).

Such switching among a plurality of blind bolometers 434 synchronouswith or otherwise consistent with switching of active bolometers 302 maynot only enable tracking of the pulse-bias heating pattern of activebolometers 302, but may also allow sufficient time for blind bolometers434 to cool down to similar temperatures as corresponding activebolometers 302 prior to being biased again. In some embodiments, eachbias column 454 may comprise the same number of blind bolometers 434 asthe number of rows of active bolometers 302 in bolometer circuit 400A(e.g., R₁=R₂= . . . =R_(B)=N). That is, for each row of active bolometer302, there is provided a corresponding blind bolometer 434 in each biascolumn 454 according to such embodiments. Such embodiments may allowblind bolometers 434 to mirror the pulse bias heating and cooling timingof corresponding active bolometers 302.

In one example, bias columns 454 together with buffer 458 may berepresented by bias circuit 108 in FIG. 1. In another example, blindbolometers 434 and associated switches 433 and 435 may be represented bycells (e.g., as columns next to unit cell array 104 rather than rowsshown above unit cell array 104) of blind bolometers 134 in FIG. 1,while the remaining portion of bias columns 454 together with buffer 458may be represented by bias circuit 108 in FIG. 1.

Remaining portions of bolometer circuit 400A may be implemented inaccordance with various embodiments and alternatives described above inconnection with FIGS. 1 through 3. For example, in embodiments ofbolometer circuit 400A, a LPF 422 may be implemented with a switchedcapacitor circuit comprising one or more capacitors 425A-B and one ormore associated switches 423A-B as briefly discussed with respect to LPF222 of FIG. 2. In the example shown in FIG. 4A, the switched capacitorcircuit is configured to open or close switches 423A and 423B accordingto desired timing (e.g., based on a clock signal) to achieve a desiredanalog signal filtering properties. For other embodiments, however, LPF422 for bolometer circuit 400A may alternatively be implemented using aRC network as shown for LPF 322 of bolometer circuit 300, or using othercomponents and techniques described herein. Conversely, other bolometercircuit embodiments (e.g., bolometer circuit 300) described herein mayutilize a switch capacitor circuit to implement a LPF.

In addition, in the embodiments of bolometer circuit 400A illustrated byFIG. 4A, offset adjustment circuitry 260 of circuit 200A may beimplemented as part of or in conjunction with amplifier 418 as brieflydiscussed above in connection with FIG. 2. In particular, amplifier 418according to various embodiments may be configured to make adjustmentsto the voltage levels at its inputs 417 and/or 421 and/or otheroperating characteristics of amplifier 418 in response to offsetadjustment bits 464 received from calibration data memory 309 for acorresponding column circuit block 414 or a pixel therein. Amplifier 418for the various embodiments illustrated by FIG. 4A may thus allowper-column or per-pixel fine adjustment of biases across activebolometers 302, for example.

As discussed above in connection with offset adjustment circuitry 260,embodiments of bolometer circuit that provide offset adjustmentcircuitry 260 as part of or in conjunction with amplifier 418 maybeneficially allow a larger portion of the supply voltage to beavailable for biasing active bolometer 302. For other embodiments,offset adjustment circuitry 260 for bolometer circuit 400A mayalternatively be implemented using ODAC 360 or other components andtechniques described herein. Conversely, other bolometer circuitembodiments described herein may utilize techniques described foramplifier 418 to implement offset adjustment circuitry 260.

As may be appreciated from the foregoing discussion, various embodimentsof bolometer circuit 400A described above may not only provideappropriate biases to active bolometers 302 with reduced complexity,size, cost, and noise compared with conventional bolometer circuits, butalso vary the biases in response to self-heating of active bolometers302 to correct for undesirable effects of the self-heating, therebyadvantageously increasing the usable range (or signal swing) of anoutput signal indicative of incident IR radiation at active bolometers302.

FIG. 4B illustrates a bolometer circuit 400B having multiple columns androws of active bolometers 302 (e.g., arranged in a FPA) in accordancewith another embodiment of the disclosure. Bolometer circuit 400B issimilar to bolometer circuit 400A, but is configured to reduce noiserelative to bolometer circuit 400A. In particular, bolometer circuit400B includes a transistor 470 (e.g., a MOSFET 470 according to someembodiments) between thermally shorted bolometer 416 and a node (e.g.,node labeled “Col” in FIG. 4B) that is coupled to input 417 of amplifier418. Thermally shorted bolometer 416 and transistor 470 connected inseries in this manner according to one or more embodiments may also bereferred to herein as a resistive load for each column circuit block414.

The source of transistor 470 (e.g., the source of MOSFET 470) is set toa voltage level relative to its gate, thereby producing a voltagedifference from supply voltage node 308 across thermally shortedbolometer 416. This voltage difference across the thermally shortedbolometer 416 generates a load current through active bolometer 302. Inthis regard, thermally shorted bolometer 416 and transistor 470connected in series may essentially operate as a current source thatgenerates the load current. Similarly, in one or more embodiments,bolometer circuit 400B may include a transistor 472 (e.g., a MOSFET 472according to some embodiments) between thermally shorted bolometer 466and a node (e.g., node labeled “BCol” in FIG. 4B) that is coupled toinput 421 of amplifier 418. Thermally shorted bolometer 466 andtransistor 472 connected in series may similarly operate as a currentsource that generates a similar load current to blind bolometers 434 foreach bias column. By providing transistors 470 and 472, thermallyshorted bolometer 416 and 466 are isolated from nodes Col and BCol,respectively, thereby reducing noise from thermally shorted bolometers416 and 466.

The voltage across active bolometer 302 is defined by the resistance ofactive bolometer 302 and the load current generated by thermally shortedbolometer 416 and transistor 470. Similarly, the voltage across blindbolometer 434 is defined by the resistance of blind bolometer 434 andthe load current generated by thermally shorted bolometer 466 andtransistor 472. Thus, as these load currents are increased, the biasesacross active bolometer 302 and blind bolometer 434 may also beincreased. With the load current providing a desired bias level (e.g., abias current) across active bolometer 302, a voltage level may bedetermined (e.g., at node Col) in response to the load current flowingthrough active bolometer 302 that exhibits a resistance change due tothe external IR radiation. With the load current providing a desiredbias level (e.g., a bias current) across blind bolometer 434, areference voltage level to input 421 of amplifier 418 may be determined(e.g., at node BCol) in response to the load current flowing throughblind bolometer 434 that tracks self-heating of active bolometer 302.MOSFETs 470 and 472 may be implemented as pMOS transistors according toone or more embodiments

In the illustrated embodiment, the gates of MOSFETs 470 and 472 may beconnected to a variable voltage source, such as a DAC 476, that providesa variable voltage level to the gates in response to adjustment bits.Thus, by controlling the variable voltage level provided by the variablevoltage source to the gates of MOSFETs 470 and 472, coarse or fineadjustments to the load currents, and thus to the active bolometer biasand gain, may be made. In addition, by having the transistors (e.g.,MOSFETs 470 and 472), the gain from the voltage at input 421 to thecurrent at input 417 of amplifier 418 may be reduced, which in turnbeneficially reduces noise from amplifier 418. Therefore, bolometercircuit 400B according to one or more embodiments may reduce noise fromamplifier 418 as well as from thermally shorted bolometers 416 and 466.

In one or more embodiments, thermally shorted bolometers 416 and 466 mayhave an increased resistance and a commensurate increase in a supplyvoltage level (e.g., a voltage level at supply voltage node 308) toreduce noise relative to bolometer circuit 400A, in addition to thenoise reduction benefit by isolating thermally shorted bolometers 416and 466 with transistors 470 and 472. In general, according to variousembodiments, the ratio of the resistance of thermally shorted bolometers416 to active bolometers 302 in column circuit blocks 414 may be largerthan that for bolometer circuit 400A, such that the supply voltage levelfor bolometer circuit 400B may be increased over the nominal supplyvoltage level used in bolometer circuit 400A (in which thermally shortedbolometers 316 and active bolometers 302 have a smaller resistance ratioas discussed above) while maintaining a similar load current to activebolometers 302 as in bolometer circuit 400A. For example, if theresistance ratio of thermally shorted bolometers 316 to activebolometers 302 in the nominal case of bolometer circuit 400A is K (e.g.,the resistance of thermally shorted bolometer 316 is K×Rb where Rbdenotes the resistance of active bolometer 302 as discussed above forFIG. 3), the resistance ratio of thermally shorted bolometers 416 toactive bolometers 302 in bolometer circuit 400B may be L, where L>K(e.g., the resistance of thermally shorted bolometer 416 is L×Rb).

In the non-limiting, illustrated embodiment of FIG. 4B, the resistanceof thermally shorted bolometers 416 is larger than the resistance ofactive bolometer 302 (i.e., the resistance ratio of thermally shortedbolometers 416 to active bolometer 302 is larger than 1), as opposed tobolometer circuit 400A where thermally shorted bolometers 316 and activebolometers 302 have a similar resistance (i.e., the resistance ratio ofthermally shorted bolometers 316 to active bolometer 302 isapproximately 1). Similarly, the increased supply voltage level forbolometer circuit 400B allows the resistance of thermally shortedbolometers 466 for bias columns 454 to be larger than the resistance ofblind bolometers 434 while providing a similar load current to blindbolometers 434 as in bolometer circuit 400A. In some embodiments,bolometer circuit 400B may include thermally shorted bolometers 416 and466 having a resistance that is two to four times that of activebolometers 302 and blind bolometers 434. In the illustrated embodiment,the resistance of thermally shorted bolometers 416 and 466 areapproximately four times (indicated in FIG. 4B as “4×Rb”) the resistanceof active bolometers 302 and blind bolometers 434. With an appropriateincrease in the supply voltage level, the load current flowing from theresistive loads (e.g., thermally shorted bolometers 416 and 466) toactive bolometers 302 and blind bolometers 434 may be substantiallysimilar to the nominal load current in bolometer circuit 400A. At thesame time, because of the resistance increase of the resistive loads(e.g., thermally shorted bolometers 416 and 466), the current noise fromthe resistive loads will be decreased, thereby obtaining a noisereduction benefit.

FIG. 4C illustrates a bolometer circuit 400C having multiple columns androws of active bolometers 302 (e.g., arranged in a FPA) in accordancewith yet another embodiment of the disclosure. Bolometer circuit 400C issimilar to bolometer circuit 400B, but has a reduced number of bolometerstructures relative to bolometer circuit 400B by configuring amplifiers418 as integrating amplifiers in place of the feedback amplifierconfiguration of bolometer circuit 400B. In particular, bolometercircuit 400C includes, for each column circuit block 414, a capacitor484, which connects output 419 to input 417 of amplifier 418, and abuffer 480 and a resistor 482 (also labeled “Rint” in FIG. 4C) inseries, which together couple a node (labeled “Col” in FIG. 4C) in thebolometer conduction path between MOSFET 470 and active bolometer 302 toinput 417 of amplifier 418, so that the difference in current throughresistor 482 is integrated onto capacitor 484. More specifically, basedon the load current created with the combination of MOSFET 470 andthermally shorted resistor 416, a voltage is generated across the activebolometer 302 based on the nominal active bolometer resistance as wellas external IR radiation incident on active bolometer 302 as discussedabove. Similarly, blind bolometers 434 creates a reference voltage levelon Bcol as a similar load current from MOSFET 472 and thermally shortedresistor 466 is passed through blind bolometers 434 as also discussedabove. The voltage level set in response to the load current flowingthrough active bolometer 302 (which exhibits the resistance change dueto the external IR radiation) is received by and passed through buffer480 to one end of resistor 482, while the reference voltage level (e.g.,at Bcol) from bias columns 434 is received by and passed through buffer458 to input 421 and maintained at input 417 of amplifier 418, and henceat the other end of resistor 482, by the use of a virtual ground asdiscussed above. The difference in these two voltage levels placedacross resistor 482 generate a current flow that is dependent on theexternal IR radiation incident on active bolometer 302, and that currentflow is integrated by amplifier 418 and capacitor 484.

Thus, in embodiments of bolometer circuit 400C, capacitor 484 replacesthermally shorted bolometer 320 operating as a resistive gain in thefeedback amplifier configuration of bolometer circuit 400B. Thispotentially increases yield, since bolometers, which are in generalcomplex structures to fabricate on silicon, are removed. Furthermore,removal of thermally shorted bolometers 320 may facilitate optimizationof the size and/or layout for bolometer circuit 400B on a silicon diefor fabrication, since thermally shorted bolometers 320 for the feedbackamplifier configuration generally take up a large part of a particularlocation of a die to fabricate.

In the illustrated embodiment, resistor 482 is provided and connected toinput 417 of amplifier 418, thereby forming an RC integrator amplifierconfiguration to provide a desired gain, reduce noise bandwidth, andreduce the effects of pulse bias heating on the output signal. In theillustrated embodiment, buffer 480 is provided between resistor 482(Rint) and the Col node in order to replicate the voltage on the Colnode without corrupting the current through the bolometer conductionpath. However, in other embodiments, buffer 480 and/or resistor 482 maybe omitted to reduce the size of the die further. These additionalcomponents, namely buffer 480, and resistor 482, may introduceadditional noise. However, as discussed above for bolometer circuit 400Band also applicable to bolometer circuit 400C, the noise reductionbenefit from having a high resistance thermally shorted bolometer 416and a high supply voltage may largely offset the additional noise by theadditional components.

For example, calculations carried out by the inventors in connectionwith the present disclosure suggest only some insignificant increase inoverall noise relative to bolometer circuit 400A of FIG. 4A. In anotheraspect, the additional areas taken up by MOSFET 478, buffer 480,resistor 482, and capacitor 484 may also be offset by the areas freed upand the layout flexibility gained by removing thermally shortedbolometers 320 of bolometer circuit 400A/400B. Therefore, any of theembodiments described with reference to FIGS. 4A-4C that meets thedesired design goals, constraints, and tradeoffs may be utilized toprovide a low cost, high performance bolometer circuit. Amplifier 418,capacitor 484, resistor 482, and/or buffer 480 may be referred to hereinas belonging to an amplifier circuit for each column circuit block 414of bolometer circuit 400C.

FIG. 4D illustrates a bolometer circuit 400D comprising circuit 200B ofFIG. 2B for multiple columns and rows of active bolometers 302 (e.g.,arranged in a FPA) to enable a low-power detection mode of operation inaccordance with an embodiment of the disclosure. As shown, in bolometercircuit 400D according to one or more embodiments, each column circuitblock 414 (e.g., for each column of a bolometer FPA) includes switches240-244 (SW1-SW5) discussed above for circuit 200B, which are associatedwith thermally shorted bolometer 316, amplifier 418, and activebolometers 302 for the column. Thus, in embodiments of bolometer circuit400D, each column circuit block 414 can individually be operated in thelow-power mode or the normal mode discussed above for circuit 200B.

In addition, in embodiments in which one or more bias columns 454 areprovided to implement variable voltage source 250 to compensate forbolometer self-heating, each bias column 454 may individually beoperated in the low-power mode or the normal mode. In this regard, eachbias column 454 includes switches 445, 446, and 453 (also labeled “SW6”through “SW8” in FIG. 4D) associated with the reference conduction path.In the illustrated embodiment, switch 445 (SW6) is configured toselectively connect or disconnect thermally shorted bolometer 456 (whichoperates as a resistive load for the reference conduction path) to orfrom supply voltage node 308, while switch 446 (SW7) is configured toselectively connect or disconnect blind bolometer 434 to or from commonvoltage node 310. Thus, by selective opening and closing of switches 445and 446, the reference conduction path (e.g., including thermallyshorted bolometer 456 and blind bolometer 434) can be connected to ordisconnected from power. As discussed above for FIG. 4A, switch 453(also referred to as column switch 453) is configured to selectivelyconnect or disconnect a corresponding bias column 454 to or from inputs421 of amplifiers 418 (e.g., via buffer 458) in bolometer circuit 400D.

In one embodiment, each bias column 454 may be operated in a low-powermode by setting switches 445 and 446 (SW6 and SW7) in an open positionto disconnect the bias column 454 (the reference conduction path) frompower. In another embodiment, switch 453 (SW8) may be opened todisconnect the bias column 454 from inputs 421 of amplifiers 418 (e.g.,via buffer 458) to operate in a low-power mode, in addition todisconnecting the bias column 454 from power by opening switches 445 and446.

In various embodiments, timing and control circuit 332 and/or processoror other logic device 340 of bolometer circuit 400D may be configured togenerate control signals to selectively open or close switches 240-244,445, 446, and 453 to operate each column circuit block 414 and biascolumn 454 in a normal mode or a low-power mode. In one embodiment,timing and control circuit 332 and/or processor or other logic device340 may be configured to operate a few selected columns (e.g., one ormore selected column circuit blocks 414) of bolometer circuit 400D in anormal mode while operating the remainder of columns in a low-power modeto capture an infrared image frame. Similarly, in case one or more biascolumns 454 are provided in bolometer circuit 400D, a few selected biascolumns 454 may be operated in a normal mode while the remainder of biascolumns 454 may be operated in a low-power mode to provide a referencevoltage level. In one non-limiting example, only 10% of column circuitblocks 414 and bias columns 454 (e.g., 16 out of 160 columns in a FPAand 1 out of 10 bias columns) may be operated in a normal mode while therest are operated in a low-power mode to save power.

Such a mode of operation may also be referred to herein as a low-powerdetection mode, which reduces the power consumed by bolometer circuit400D but at the same time permits bolometer circuit 400D to be used todetect changes in the scene (e.g., changes in external infraredradiation from the scene) using those columns of bolometer circuit 400Dthat operate in a normal mode. For example, in one embodiment, timingand control circuit 332 and/or processor or other logic device 340 maybe configured to “wake up” bolometer circuit 400D in response todetecting a change in the scene while in low-power detection mode, byswitching from a low-power detection mode to a “normal imaging mode”that operates all columns of bolometer circuit 400D in a normal mode tocapture full infrared image frames. Thus, bolometer circuit 400D may beoperated in a low-power detection mode to save power while there is noactivity of interest in the scene being imaged (e.g., no or littlechange in the infrared radiation emitted from the scene), but switchedto a normal imaging mode to capture infrared image frames of the sceneupon detection of an activity (e.g., changes in the infrared radiationemitted from the scene).

In some embodiments, when operating bolometer circuit 400D in alow-power detection mode to capture a series of infrared image frames,timing and control circuit 332 and/or processor or other logic device340 may be configured to cycle through column circuit blocks 414 andbias columns 454 to select the few column circuit blocks 414 and biascolumns 454 to operate in a normal mode. That is, those column circuitblocks 414 and bias columns 454 that operate in the normal mode areselected in a round-robin manner from all column circuit blocks 414 andbias columns 454 of bolometer circuit 400D as each IR image frame iscaptured. In a non-limiting example given for purposes of illustration,a bolometer circuit may have 160 active columns (e.g., column circuitblocks 414 with active bolometers 302) and 10 bias columns and use 10%of the active and bias columns in a low-power detection mode, where biascolumn 1 and active columns 1 through 16 (column numbers are given forpurposes of identification only) may be operated in a normal mode in thefirst image frame captured, bias column 2 and active columns 17-32 inthe second image frame, bias column 3 and active columns 33-48 and so onuntil the 11th image frame when the cycle restarts. Cycling throughcolumn circuit blocks 414 and bias columns 454 in such a manner maybeneficially prevent a “burn-in effect” of having components of somecolumn circuit blocks 414 (e.g., active bolometers 302 associated witheach column circuit block 414) and bias columns 454 (e.g., blindbolometers 434 associated with each bias column 454) being used morethan those of others, if bolometer circuit 400D is frequently orregularly operated in a low-power detection mode. This cycling throughcolumn circuit blocks 414 and bias column 454 in a round-robin manneralso allows all bolometers to keep approximately the same temperaturedue to pulse-biased heating, so that when a switch from low-power modeto normal operation occurs, there is minimal time needed to get allbolometers in the array back to the same temperature to obtain a uniformimage.

Therefore, embodiments of bolometer circuit 400D may be operated in alow-power detection mode to provide advantageous power saving whilewaking up to capture full infrared image frames when needed. Althoughbolometer circuit 400D is illustrated in FIG. 4D as having bolometercircuit 400A as the base architecture, it should be appreciated that thetechniques and principles described in connection with embodiments ofbolometer circuit 400D are also applicable to other bolometer circuitshaving an active bolometer, a resistive load, an amplifier, and/or othercomponents generally found in bolometer circuits. For example, abolometer circuit, which enables a low power mode and/or a low-powerdetection mode of operation according to the principles and techniquesdiscussed for various embodiments of circuit 200B and bolometer circuit400D, may be provided with respect to bolometer circuits 300, 400B, and400C discussed above, bolometer circuits 500 and 800 discussed hereinbelow, or any other suitable bolometer circuit. Furthermore, although alow-power detection mode is illustrated above as operating an entirecolumn (e.g., including all active bolometers 302 in a column circuitblock 302) in a low-power mode, it is also contemplated for otherembodiments that a low-power detection mode may be operated in aper-pixel basis, where only one or more selected active bolometers 302of a column are operated in a low-power mode instead of the entirecolumn.

Other techniques to reduce the effects of active bolometer self-heatingor pulse bias heating are also contemplated. For example, FIG. 5Aillustrates a bolometer circuit 500 in which two adjacent rows of activebolometers may be enabled and similarly biased to generate an outputsignal with reduction in undesirable effects of self-heating, inaccordance with an embodiment of the disclosure.

According to various embodiments, bolometer circuit 500 may comprise oneor more column circuit blocks 514(1) through 514(M) each associated witha plurality of active bolometers 502. In FIG. 5A, only those activebolometers 502(1) through 502(N) associated with column circuit block502(1) is shown for clarity, but it will be understood that other activebolometers 502 not shown in FIG. 5A may be provided and associated withother column circuit blocks 514(2)-514(M). For example, there may beprovided M×N active bolometers 502 in bolometer circuit 500, with N rowsof active bolometers 502 per each of M column circuit blocks 514,according to some embodiments.

According to various embodiments, two adjacent ones of active bolometers502 (e.g., active bolometers 502(1) and 502(2) and associated circuitryshown in detail in FIG. 5A) may be selectively connected and enabledsuch that the two adjacent active bolometers may form a circuit pathcoupled to supply voltage node 308 at one end and to ODAC 360 at theother end, while coupling a node between the two adjacent activebolometers (e.g., a node 503(1) identified in FIG. 5A between activebolometers 502(1) and 502(2)) to an input (e.g., inverting input 317) ofamplifier 318. In this regard, a plurality of switches 580, 582, and 584(e.g., switches 580(1)-580(3), 582(1)-582(3), and 584(1)-584(3) shown indetail in FIG. 5A) may be provided and selectively opened and closedaccording to control signals from timing and control circuit 332 asfurther described herein.

For example, when switch 580(1), 582(2), and 584(3) are closed with theremaining ones of switches 580, 582, and 584 open, active bolometer502(1) may be coupled to supply voltage node 308 and active bolometer502(2) may be coupled to ODAC 360 with node 503(1) coupled to invertinginput 317 of amplifier 318 (e.g., via a node 505 designated “Col” inFIG. 5A to route a difference of pixel signals between active bolometers502(1) and 502(2), as further discussed below). In a similar manner,pairs of active bolometers 502(3) and 502(4) through 502(N−1) and 502(N)may be selectively enabled and connected to column circuit block 502(1)according to specified timing by control signals from timing and controlcircuit 332. In some contexts, active bolometer coupled to supplyvoltage node 308 (e.g., as in active bolometer 502(1) in this example)may be referred to as a top detector of the pair, and active bolometercoupled to common voltage node 310 directly or via ODAC 360 (e.g., as inactive bolometer 502(2) in this example) may be referred to as a bottomdetector in the pair.

Thus, in various embodiments of bolometer circuit 500, a top detector(which is an active bolometer receiving incident IR radiation) of aselected pair of adjacent active bolometers 502 may take the place ofthermally shorted bolometer 216 or 316 that operates as a non-activeload (e.g., non-responsive to incident IR radiation) in bolometercircuit 200A-B, 300, or 400A-B. In other aspects, except for a lack ofthermally shorted bolometer 316, each of the column circuit blocks514(1)-514(M) may comprise similar components as column circuit blocks314 or 414. Consequently, with appropriate biases applied across activebolometers 502(1) and 502(2), an amplified output voltage at output 319of amplifier 318 for bolometer circuit 500 may correspond to adifference in the intensity of incident IR radiation (e.g., a differencein pixel signals) between the bottom detector (e.g., active bolometer502(2)) and the top detector (e.g., active bolometer 502(1)) in a pairof adjacent active bolometers 502, rather than corresponding to someabsolute level of incident IR radiation received at each activebolometer 202, 302, or 402 as provided by embodiments of bolometercircuit 200A-B, 300, or 400A-D. In this sense, bolometer circuit 500 maybe referred to as implementing a differencing architecture or differenceimaging architecture, whose output signals may represent images in adifference domain (e.g., where each pixel value represents a differencein incident IR radiation intensity between adjacent detectors) asopposed to a direct image domain (e.g., where each pixel value isindicative of the intensity of IR radiation received at each detector),for example.

Advantageously, using a pair of adjacent active bolometers 502 in anarrangement according to various embodiments of bolometer circuit 500reduces the undesirable effects of self-heating or pulse bias heating onoutput signals, because both of the active bolometers in a selected pair(e.g., active bolometers 502(1) and 502(2)) exhibit similar self-heatingor pulse bias heating such that the effects of self-heating of the pairare naturally canceled. Further, effects of ambient temperaturevariations are well compensated for in embodiments of bolometer circuit500, because adjacent active bolometers 502 in a pair are typically in aclose physical proximity to each other in an FPA and thus exposed tosimilar ambient temperature conditions. Yet another advantage is thatadjacent ones of active bolometers 502 typically exhibit littlevariation in operating characteristics such as their resistance, TCR,heat capacity, and thermal conductivity, and thus require little or noadjustment to compensate for variations in bolometer operatingcharacteristics, whereas such adjustments may be needed when separatethermally shorted bolometers 216 or 316 are used as a load.

Various embodiments of bolometer circuit 500 may therefore beneficiallypermit removal or reduction of various components and circuitryconventionally required to compensate for self-heating, bolometervariations, and/or ambient temperature variations, thereby allowingfurther reduction in complexity, size, and cost. Furthermore, becausevarious embodiments of bolometer circuit 500 obtains a local difference,rather than a direct measurement, of IR radiation intensity, a highscene dynamic range can be achieved. In other words, a differenceimaging architecture implemented according to one or more embodiments ofbolometer circuit 500 may beneficially allow an image of a scene havinga wider range of IR radiation intensity to be captured. In this regard,the intensities of incident IR radiation received at adjacent ones ofactive bolometers 502 are often similar (e.g., due the diffraction limitof long wavelength IR radiation through optical elements and otherfactors that smear or blur incident IR radiation onto neighboring onesof active bolometers 502), which further facilitates capturing of highscene dynamic range by differencing architectures.

As further described herein below, images in a difference domainobtained with one or more embodiments of bolometer circuit 500 may bereconstructed into images in a direct image domain, if desired forparticular applications of bolometer circuit 500. For example, if IRradiation from a scene captured by embodiments of bolometer circuit 500is to be presented for viewing and easy understanding by a human user,difference images obtained using bolometer circuit 500 may bereconstructed (e.g., converted) into images (e.g., thermograms) in adirect image domain where each pixel value corresponds to IR radiationintensity received at each detector. On the other hand, for video/imageanalytics or other image processing applications, for example,difference images may be sufficient or even beneficial, and thus wouldnot need to be reconstructed into direct images.

In this regard, for some embodiments, bolometer circuit 500 may includeone or more reference rows for facilitating accurate and efficientreconstruction of difference images into direct images. For example, inone or more embodiments, bolometer circuit 500 may include one or morethermally shorted bolometers, blind bolometers, thermopiles, and/orother temperature sensing components to provide ambient temperatures orambient IR intensity levels that can be used as reference points inreconstruction (e.g., conversion) of difference images into directimages.

In embodiments illustrated by FIG. 5A, one or more blind bolometers 534may be provided for each column circuit block 502 (in FIG. 5A, onlythose blind bolometers 534(1) through 534(R) associated with one columncircuit block 502(1), where R may represent the desired number of blindbolometer rows in bolometer circuit 500, are shown for clarity), andconfigured to be selectively enabled and connected via associatedswitches 586, 587, and 588 to input 317 of amplifier 318 and to one ofactive bolometers 502 associated with the each column circuit block 514.For example, blind bolometer 534(1) may be connected to supply voltagenode 308 and to node 505 (labeled “Col” in FIG. 5A) by closing switches586 and 587 (e.g., based on control signals from timing and controlcircuit 332) while active bolometer 502(1) may be connected to commonvoltage node 310 (via ODAC 360) and to node 505 (Col) by closing ofswitches 584(1) and 582(2). When selectively enabled and connected as inthis example, a difference between a current flow through activebolometer 502(1) due to incident IR radiation and a current flow throughblind bolometer 534(1) due to ambient temperature (since blindbolometers are shielded from incident IR radiation) may be amplified andconverted into an output voltage level at output 319 of amplifier 318according to the principles described above with respect to FIGS. 2 and3, thereby providing an output signal indicative of incident IRradiation intensity received at active bolometer 502(1) over a referencelevel.

In a similar manner, blind bolometers 534 may be selectively enabled andconnected as references for one or more other active bolometers 502,depending on embodiments. In one specific example according to one ormore embodiments, there may be provided two blind bolometers 534 percolumn, one to provide a reference for an active bolometer on the toprow (e.g., active bolometer 502(1)) and the other for an activebolometer on the bottom row (e.g., active bolometer 502(N)), asillustrated by FIG. 5B. In another specific example according to one ormore embodiments, there may be provided four blind bolometers 534(1)through 534(4) per column, two for the top and bottom rows of activebolometers 502 as in the previous example and the other two for twomiddle rows of active bolometers 502, as illustrated by FIG. 5C. Inanother specific example according to one or more embodiments, there maybe provided blind bolometers 534 for every two rows in the quartiles ofall active bolometers 502 provided per column, in addition to the twofor top and bottom rows of active bolometers 502, as illustrated by FIG.5D. That is, if assuming the number of active bolometers 502 per columnis 16 (e.g., N=16), there may be provided eight blind bolometers 534(1)through 534(8) respectively associated with active bolometers 502(1),502(4), 502(5), 502(8), 502(9), 502(12), 502(13), and 502(16), forexample.

Further details of active bolometers 502, blind bolometers 534, andassociated switching logic according to one or more embodiments of thedisclosure are discussed with reference to FIGS. 6A and 7A-7D. FIG. 6Aillustrates a circuit portion 600A, which may represents a portion ofbolometer circuit 500, comprising rows of active bolometers 502, rows ofblind bolometers 534, and accompanying sets of switches 580, 582, 584,586, 587, and 588 that may be associated with any one of column circuitblocks 514, in accordance with an embodiment of the disclosure. FIGS.7A-7D illustrate how difference image frames may be obtained withembodiments of bolometer circuit 500, in accordance with an embodimentof the disclosure.

As shown in FIG. 6A, portion 600A of bolometer circuit 500 may comprisea first set of switches 580(1) through 580(N+1), a second set ofswitches 582(1) through 582(N+1), and a third set of switches 584(1)through 584(N+1) for selectively enabling and connecting activebolometers 502(1) through 502(N) to obtain an output signal indicativeof a difference in IR radiation intensity between to two adjacent (e.g.,neighboring or adjoining) ones of active bolometers 502(1) through502(N), according to one or more embodiments. The first set of switches580 may be closed to connect a corresponding one of active bolometers502 to supply voltage node 308. In this sense, the first set of switches580 may also be referred to as “Vload Enable” switches (labeled in FIG.6A as VLDEN(1) through VLDEN(N+1)). The second set of switches 582 maybe closed to connect a corresponding one of active bolometers 502 toinput 317 of amplifier 318 via node 505 (labeled as “Col” in FIGS. 5Aand 6A) of column circuit block 514. In this sense, the second set ofswitches 582 may also be referred to as “Column Enable” switches(labeled in FIG. 6A as COLEN(1) through COLEN(N+1)). The third set ofswitches 584 may be closed to connect a corresponding one of activebolometers 502 to common voltage node 310, via ODAC 360 or not dependingon embodiments. In this sense, the third set of switches 584 may also bereferred to as “Common Enable” switches (labeled in FIG. 6A as COMEN(1)through COMEN(N+1)).

As discussed above with reference to FIG. 5A, switch 580(1) (orVLDEN(1)), switch 582(2) (or COLEN(2)), and switch 584(3) (or COMEN(3))may be closed while the remaining ones of switches 580, 582, and 584 maybe opened in response to control signals from timing and control circuit332 to obtain a difference signal (e.g., an output signal indicative ofthe intensity of incident IR radiation received at active bolometer502(2) over that received at active bolometer 502(1)). According to oneor more embodiments, timing and control circuit 332 and/or processor orother logic device 340 of bolometer circuit 500 may be configured togenerate control signals to repeat such selective enabling andconnection for obtaining a difference signal for every even row in eachcolumn of active bolometers 502 in a sequential manner. In this way, aframe (e.g., an image frame having columns and rows) may be obtainedwhich comprises difference signals (e.g., digital or analog signals ordata values) indicative of incident IR radiation intensity levels at rowi minus those at row i−1, where i=2, 4, 6, . . . , N. Such a frame mayalso be referred to herein as an “even-down” difference frame, since itcontains signals indicative of incident IR radiation intensity at evennumbered rows over the respective odd numbered rows that precede them.

An example of how such an even-down difference frame may be obtained(e.g., sampled and/or read out by embodiments of bolometer circuit 500)is illustrated in FIG. 7A, in accordance with an embodiment of thedisclosure. In FIG. 7A, an array of active bolometers 502 and two rowsof blind bolometers 534 are represented by blocks, while curved arrowsindicate which rows of active bolometers 502 or blind bolometers 534 arebeing compared to generate difference signals and the direction (e.g.,direction from the top detector to the bottom detector in a pair) of thecomparison. Thus, as indicated in FIG. 7A, an even-down difference framemay be obtained by selectively enabling and connecting active bolometers502 via sets of switches 580, 582, and 584 to generate difference rowscomprising signals indicative of incident IR radiation intensity at eacheven row minus that at a corresponding odd row that precedes it.

In various embodiments, timing and control circuit 332 and/or processoror other logic device 340 of bolometer circuit 500 may be configured togenerate further control signals for sets of switches 580, 582, and 584to selectively enable and connect active bolometers 502 to obtain anodd-down difference frame which comprises difference signals indicativeof incident IR radiation intensity at odd numbered rows over therespective even numbered rows that precede them. An example of how suchan odd-down difference frame may be obtained is illustrated in FIG. 7B,in accordance with an embodiment of the disclosure.

As illustrated in FIG. 7B, the first difference row in an odd-downdifference frame may be obtained by selectively enabling and connectinga row of blind bolometers 534 (e.g., Blind Reference Row 1 in FIG. 7B)for comparison 702 with the first row (e.g., Row 1 in FIG. 7B) of activebolometers 502. As further discussed herein, the first difference rowmay thus comprise signals indicative of incident IR radiation intensityat the first row of active bolometers 502 in some absolute terms (e.g.,over a reference temperature level provided by the row of blindbolometers 534), which may beneficially be used, for example, toaccurately reconstruct difference image frames into direct images. Inthis sense, the first difference row, as well as other difference rowsthat may represent incident IR radiation intensity in absolute terms,may also be referred to as an absolute measurement row.

In some embodiments, an odd-down difference frame may include an extradifference row (e.g., as the last difference row) based on comparisons704 between another row of blind bolometers 534 (e.g., Blind ReferenceRow 2 in FIG. 7B) and the last row (e.g., Row N in FIG. 7B) of activebolometers 502. The extra difference row may thus comprise signalsindicative of incident IR radiation intensity at the last row of activebolometers 502 in absolute terms. As further discussed herein, signalsin absolute terms provided by the extra difference row (or the extraabsolute measurement row) may be used to estimate and/or reduce varioustypes of noise in reconstructed direct images.

For the remaining rows i=3, 5, 7, . . . , N−1 of active bolometers 502,corresponding difference rows are obtained which comprise differencesignals indicative of incident IR radiation intensity at row i minusthat at row i−1. Thus, an odd-down difference frame and an even-downdifference frame may together comprise difference rows obtained for alladjacent rows of active bolometers 502, with the first and the lastdifference rows being absolute measurement rows comprising signalsmeasured in absolute terms. In this regard, in one or more embodiments,an odd-down difference frame may be obtained immediately following aneven-down difference image, or vice versa, to capture difference signalsbetween all adjacent rows of active bolometers 502 in an interlacedmanner. In one aspect, such interlaced capturing may beneficially allowfor better matching of self-heating between two adjacent rows of activebolometers 502. For example, adjacent rows of active bolometers 502,when they are to be selectively connected and biased to obtain odd-down(or even-down) difference signals, will have had similar time to cooldown (e.g., only differing by one row-time) after having been biased andself-heated in a previous even-down (or odd-down) difference frame, andthus exhibit similar self-heating characteristics (e.g., startingtemperatures and/or heat-up rate). In comparison, adjacent rows ofactive bolometers 502 will not have had similar time to cool down ifdifference signals for all adjacent rows were captured in one frame in aprogressive manner.

In some embodiments, additional or alternative difference frames may beobtained in which comparisons between rows of active bolometers 502 aremade in opposite directions to those made in even-down and odd-downdifference frames. For example, an even-up and an odd-up differenceframe may be obtained. According to one or more embodiments, an even-updifference frame may be obtained by selectively enabling and connectingactive bolometers 502 via sets of switches 580, 582, and 584 to generatedifference rows comprising signals indicative of incident IR radiationintensity at each even row minus that at a corresponding odd row thatfollows it. According to one or more embodiments, an odd-up differenceframe may be obtained by selectively enabling and connecting activebolometers 502 via sets of switches 580, 582, and 584 to generatedifference rows comprising signals indicative of incident IR radiationintensity at each odd row minus that at a corresponding even row thatfollows it.

More specifically, as illustrated by the example of FIG. 7C, for rowsi=2, 4, . . . , N−2 of active bolometers 502, corresponding differencerows in an even-up difference frame are obtained which comprisedifference signals indicative of incident IR radiation intensity at rowi minus that at row i+1. The first and the last difference rows in aneven-up difference frame may be based on comparisons 706 and 708,respectively, with corresponding rows of blind bolometers 534, and thusis similar to the first and the last difference rows obtained in anodd-down difference frame as illustrated in FIG. 7B except thatcomparisons 706 and 708 are made in opposite directions to comparisons702 and 704.

Also, as illustrated by the example of FIG. 7D, an odd-up differenceframe may be obtained that comprises difference rows of signalsindicative of incident IR radiation intensity levels at row i minusthose at row i+1, where i=1, 3, 5, . . . N−1. As further describedherein, an even-up difference frame and an odd-up difference frameadditionally obtained in some embodiments may be used together with aneven-down difference and an odd-down difference images to identify andreduce certain types of noise, such as spatial column noise, indifference images and/or reconstructed direct images.

Table 1 summarizes an example switching sequence to obtain even-down,odd-down, even-up, and odd-up difference frames, in accordance with anembodiment of the disclosure. As discussed above, timing and controlcircuit 332 and/or processor or other logic device 340 of bolometercircuit 500 may be configured to generate control signals to selectivelyopen or close switches 580, 582, 584, 586, 587, and 588 (e.g., alsolabeled as switches VLDEN, COLEN, COMEN, BVLDEN, BCOLEN, and BCOMEN inFIG. 6A) to generate difference signals in a sequential manner to obtaindifference frames. It should be noted that the sequence of switching inthe example is not limiting, and can be carried out in other appropriatesequences provided that appropriate switches are selectively closed andopened as indicated in Table 1 to obtain difference signals for eachpair of adjacent active bolometer rows according to desired timing.

TABLE 1 FRAME TYPE SWITCHING SEQUENCE Even-Down Difference Frame For i =2, 4, 6, . . . , N,  close VLDEN(i − 1), COLEN(i), COMEN(i + 1)  whileopening remaining switches (other switches of  VLDEN, COLEN, COMEN,BVLDEN, BCOLEN,  BCOLEN). Odd-Down Difference Frame Close BVLDEN(1),BCOLEN(1), COLEN(1), COMEN(2)  while opening remaining switches; For i =3, 5, 7, . . . , N − 1,  close VLDEN(i − 1), COLEN(i), COMEN(i + 1)while  opening remaining switches; and Close VLDEN(N), COLEN(N + 1),BCOLEN(2), BCOMEN(2)  while opening reaming switches. Even-Up DifferenceFrame Close VLDEN(2), COLEN(1), BCOLEN(1), BCOMEN(1)  while openingremaining switches; For i = 2, 4, 6, . . . , N − 2,  close VLDEN(i + 2),COLEN(i + 1), COMEN(i) while  opening remaining switches; and CloseBVLDEN(2), BCOLEN(2), COLEN(N + 1), COMEN(N)  while opening reamingswitches. Odd-Up Difference Frame For i = 1, 3, 5, . . . , N − 1,  closeVLDEN(i + 2), COLEN(i + 1), COMEN(i) while  opening remaining switches.

Therefore, in one or more embodiments, even-down, odd-down, even-up, andodd-up difference frames comprising difference signals for pairs ofadjacent rows of active bolometers 502 may be obtained with bolometercircuit 500, in accordance with the switching techniques described abovein connection with FIGS. 6A and 7A-7D and Table 1. The difference framesmay be provided as digital output signal 590 containing differencevalues in digital form for all applicable difference rows (e.g., even orodd) for all columns in each difference frame, in a manner similar toconverting and multiplexing digital output signal 390 described abovefor bolometer circuit 300.

It should be noted that not all four types of difference frames areneeded for some embodiments. As discussed herein, an even-downdifference frame and an odd-down difference frame together, or aneven-up difference frame and an odd-up difference frame together, maycapture a full set of difference rows in an interlaced manner, and thusmay be sufficient in some embodiments to produce difference imagesand/or reconstructed direct images. Other embodiments may utilize threeor four of the four types of difference images to identify and reducecertain types of noise, as discussed herein.

It should also be noted that while embodiments that obtain even and odddifference rows in an interlaced manner are illustrated by FIGS. 6A and7A-7D, the principles of the switching techniques discussed above may beapplied to obtain progressive difference frames that comprise withineach frame all difference rows for all pairs of adjacent activebolometer rows. For example, a down-progressive difference frame, andadditionally or alternatively an up-progressive difference frame, may beobtained using embodiments of bolometer circuit 500 having switchingcircuitry illustrated by circuit portion 600A. It should furthermore benoted that the four types of difference frames may be obtained in anydesired order and not limited to the order indicated in FIGS. 7A-7D.

As discussed above in connection with FIGS. 5A-5D, bolometer circuit 500for some embodiments may comprise more than or less than two rows ofblind bolometers 534 for reference rows. For such embodiments, theswitching sequence and the corresponding difference frames may bemodified appropriately in accordance with the principles of theswitching techniques disclose herein to compare the more or less thantwo blind bolometer rows with respective corresponding active bolometerrows (e.g., middle two rows of active bolometers or quartile rows ofactive bolometers for some embodiments) to obtain difference frames.Moreover, although in FIG. 6A the example switching circuitry isillustrated as having shared contacts 603(1)-603(N) between adjacentactive bolometers 502, the principles of the switching techniquesdescribed above may be modified (e.g., by repeating appropriateswitches, nodes, and/or circuit paths) for other embodiments that do nothave shared contacts between adjacent bolometers (e.g., where activebolometers 502 may have isolated contacts).

It is contemplated difference signals between adjacent columns of activebolometers 502 (also referred herein as column difference signals), inaddition to or in place of difference signals between adjacent rows, maybe captured for some embodiments. For example, a set of switches may beprovided for bolometer circuit 500 to selectively connect activebolometers 502 of adjacent columns for obtaining differences in incidentIR radiation between adjacent columns of active bolometers 502. FIG. 6Billustrates switching circuitry associated with two adjacent columns ofactive bolometers 502 of bolometer circuit 500, in accordance with anembodiment of the disclosure. In FIG. 6B, circuit portions 600B eachrepresent at least a portion of active bolometers 502 and switchingcircuitry that may be associated with one column circuit block 514 ofbolometer circuit 500. In that respect, circuit portion 600B is similarto circuit 600A of FIG. 6A, but adjacent rows of active bolometers 502in circuit portion 600B do not have shared contacts as do activebolometers 502 illustrated in circuit portion 600A.

FIG. 6B shows two such circuit portions 600B for two adjacent columns jand j+1 (i.e., any two neighboring columns) of active bolometers 502 ofbolometer circuit 500, with a set of switches 578 (individuallyidentified as switches 578(1)(j) through 578(N)(j), where N mayrepresent the desired number of rows in the unit cell array of bolometercircuit 500) configured to selectively connect corresponding pairs ofactive bolometers 502 in adjacent columns j and j+1, in accordance withan embodiment of the disclosure. Active bolometers 502 and switches 580,582, 584 are also individually identified in FIG. 6B as activebolometers 502(1)(j) through 502(N)(j) and switches 580(1)(j) through580(N)(j), 582(1)(j) through 582(N)(j), 584(1)(j) through 584(N)(j) forcolumn j, and active bolometers 502(1)(j+1) through 502(N)(j+1) andswitches 580(1)(j+1) through 580(N)(j+1), 582(1)(j+1) through582(N)(j+1), 584(1)(j+1) through 584(N)(j+1) for column j+1, but blindbolometers 534 and conduction paths to ODAC 360 and nodes 308, 310, 505are not repeated from FIG. 6A to avoid clutter.

In embodiments having switches 578 configured to selectively connectactive bolometers 502 of adjacent columns, timing and control circuit332 and/or other logic device/circuitry may be configured to generatecontrol signals to selectively open or close appropriate ones ofswitches 578, 580, 582, and 584 to obtain a difference signal indicativeof a difference in the intensity of incident IR radiation received atactive bolometers 502 of adjacent columns (e.g., between activebolometer 502(i)(j) and 502(i)(j+1)). In one example sequence of suchcontrol signals, switches 578(1)(j), 580(1)(j), 584(1)(j+1), and eitherswitch 582(1)(j) or 582(1)(j+1) may be closed while remaining ones ofswitches 578, 580, 582, and 584 may be opened to obtain a differencesignal indicative of the intensity of incident IR radiation received atactive bolometer 502(1)(j+1) over that received at active bolometer502(1)(j).

Continuing in a similar manner for this example, switches 578(i)(j),580(i)(j), 584(i)(j+1), and either switch 582(i)(j) or 582(i)(j+1) maybe selectively closed while remaining switches are selectively opened toobtain a difference signal of active bolometer 502(i)(j+1) over activebolometer 502(i)(j), where i=2, 3, . . . , N. Similar to even-down andodd-down difference frames, column difference signals between columns jand j+1 may be obtained for j=1, 3, 5, . . . , M−1 (or for j=1, 3, 5, .. . , M−2 if M is an odd number) in parallel row-by-row in one frame,and for j=2, 4, 6, . . . , M (or for j=2, 4, 6, . . . , M−1 if M is anodd number) in a following frame, where M may represent the desirednumber of columns in the unit cell array of bolometer circuit 500, forexample.

As may be appreciated, in embodiments where column difference signals,but not row difference signals, are to be obtained, bolometer circuit500 may comprise fewer column circuit blocks 514 than there are columns(e.g., one column circuit block 514 for every two columns may besufficient). It is also contemplated that blind reference columns may beprovided in a similar manner as blind reference rows illustrated inFIGS. 7A-7D but modified for column-to-column comparison. Forembodiments having blind reference columns, the switching sequence toobtain column difference signals may be modified to obtain one or moreabsolute measurement columns, in accordance with the techniquesdiscussed above in connection with FIGS. 6A-6B and 7A-7D. Further, insome embodiments, difference signals between columns may be obtained inan opposite direction (e.g., where active bolometers 502(i)(j+1) incolumn j+1 are top detectors provided with the supply voltage). Forexample, switches 578(i)(j), 580(i)(j+1), 584(i)(j), and either switch582(i)(j) or 582(i)(j+1) may be selectively closed while remainingswitches are selectively opened to obtain a difference signal of activebolometer 502(i)(j) over active bolometer 502(i)(j+1), where i=1, 2, . .. , N.

It should be appreciated that switches 578 and the switching sequencesdiscussed above in connection with FIG. 6B are mere examples for one ormore embodiments to illustrate the principles of techniques to obtaincolumn difference signals between two adjacent columns in accordancewith the present disclosure, and that various modifications are possibleand within the scope and spirit of the disclosure. For example,additional switches may be provided (e.g., by repeating one or more setsof switches at other nodes for simplification of timing and control, forimproved routing and signal quality, due to specific bolometer FPAfabrication requirements, or otherwise as desired for particularimplementations) and/or switching sequences may comprise differentorders or combinations (e.g., to selectively close two or more switchesinstead of one from a set while opening the remaining switches from theset due to additional/repeated switches), without departing from thescope of the disclosure. In another example, instead directly connectingactive bolometers 502(i)(j)_and 502(i)(j+1) in adjacent columns j andj+1 when closed as shown in FIG. 6B, switches 578 may be configured toselectively connect active bolometer 502(i)(j) in one column j to node505(j+1) (node Col(j+1)) of the other column j+1 of the adjacentcolumns, or vice versa, thereby indirectly connecting active bolometers502(i)(j) and 502(i)(j+1) in the adjacent columns when closed. Asfurther discussed herein below, column difference signals obtainedbetween two adjacent columns of active bolometers 502 may facilitatereduction of noise in images reconstructed from difference frames.

FIG. 8 illustrates a bolometer circuit 800 implementing a differencingarchitecture, in accordance with another embodiment of the disclosure.As illustrated in FIG. 8, offset adjustment circuitry 260 of circuit200A may be implemented with a multiplexer 860 together with a biascircuit 850 configured to provide a desired range of reference voltagelevels over a plurality of connections 851 in response to biasadjustment bits 852, in place of ODAC 360 implementing offset adjustmentcircuitry 260 and CMOS bias circuit 350 implementing variable voltagesource 250 for bolometer circuits 300 and 500. Bolometer circuit 800 mayotherwise be similar to bolometer circuit 500.

According to various embodiments, multiplexer 860 (e.g., implementedwith a transmission gate multiplexer in one embodiment) may beconfigured to pass a voltage level provided at a selected one of theplurality of connections 851 to input 321 of amplifier 318 in responseto offset adjustment bits 864. For example, bias circuit 850 may be setby bias adjustment bits 852 to provide voltage levels betweenV_(ref)−0.2 volt and V_(ref)+0.2 volt in 0.05 volt increments oncorresponding connections 851, while multiplexer 860 may selectivelypass one of the voltage levels (e.g., V_(ref)+0.15 volt) based on offsetadjustment bits 864. In this way, bias adjustment bits 852 may be usedto provide a coarse global bias adjustment by setting a plurality ofreference voltage levels via connections 851, one of which may beselected using offset adjustment bits 864 to provide a fine adjustmentto biases across active bolometer 502 on a per pixel pair or per columnbasis.

Furthermore, as discussed above in connection with offset adjustmentcircuitry 260, because multiplexer 860 is outside of the conduction pathof active bolometers 502 and thus does not have to operate under largecurrent variations, offset adjustment provided by multiplexer 860 may bemore stable and accurate over temperature and a larger portion of thesupply voltage may be available for biasing active bolometers 502 thanimplementations using DACs (e.g., ODAC 360) in the conduction path ofactive bolometers 502 to provide offset adjustment. It should be notedthat bias circuit 850 and multiplexer 860 are illustrated for bolometercircuit 800 as an example, and that other bolometer circuits (e.g.,bolometer circuit 300 or 400A-D) disclosed herein may also utilize biascircuit 850 and multiplexer 860 to implement variable voltage source 250and offset adjustment circuitry 260 in place of CMOS bias circuit 350,ODAC 360, and/or offset-adjustable amplifier 418. While embodimentshaving bias circuit 350/850, amplifier 318, and other associatedcomponents configured to set and maintain biases for active bolometers502 are shown above to illustrate the principles of the differencingarchitecture of the present disclosure, it should also be noted thatother embodiments are also contemplated that implement the principles ofthe differencing architecture with bolometer circuits having other biascontrol mechanisms and/or other amplifier configurations (e.g., anintegrating amplifier).

Turning now to FIG. 9, a process 900 is illustrated for processingdifference frames in accordance with an embodiment of the disclosure.For example, embodiments of process 900 may be performed on differenceframes 910 captured using various embodiments of bolometer circuit 500or 800 described above to generate images (e.g., reconstructed fromdifference frames) in a direct image domain where each pixel of theimages contain data indicative of the intensity of IR radiation receivedat each detector. In some embodiments, process 900 may also reducevarious types of noise that may be present in difference frames and/orreconstructed direct images.

Depending on embodiments, process 900 may be performed on differenceframes 920 that include an even-down difference frame and an odd-downdifference frame consecutively obtained in an interlaced manner, aneven-up difference frame and an odd-up difference frame consecutivelyobtained in an interlaced manner, or three or four of all four types ofdifference frames consecutively obtained with bolometer circuit 500 or800. As discussed above, in one or more embodiments, difference frames910 may be provided in a digital format representing difference valuesdigitally for all corresponding difference rows (e.g., even or odd) forall columns in each difference frame.

In some embodiments, various operations to reduce column noise indifference frames 910 may be performed at block 920. In this regard, theinventors of the present disclosure have recognized, through variousexperiments performed in connection with the disclosure, that noise indifference frames exhibits similar properties as those exhibited bynoise in direct images captured by non-differencing bolometer circuits(e.g., bolometer circuits 300 or 400A-D), but that additional types ofnoise may be introduced or otherwise present after difference frames arereconstructed into direct images. Thus, the inventors have devisedcertain noise reduction operations that may be performed in a differencedomain prior to reconstruction into direct images, as well as certainother noise reduction operations that may be performed in a directdomain after reconstruction to reduce additional types of noiseintroduced or otherwise present in reconstructed direct images.

Further in this regard, the inventors have recognized, through variousexperiments, that column noise in difference frames may be at leastpartly uncorrelated among different types of difference frames 910.Thus, at block 920, column noise reduction may be performed separatelyfor each type of difference frame so as to avoid blending of anyuncorrelated column noise among different types of difference frames910. An example column noise reduction process 1000 that may bepreformed at block 920 is illustrated according to an embodiment of thedisclosure, with reference also to FIG. 10.

As understood in the art, column noise is a type of noise that may beexplained by variations in per-column circuitry and manifest as verticalstripes (e.g., effecting an entire column) in images. Further, someportion of column noise may be spatial (e.g., not substantially varyingover time) or 1/f (e.g., slowly varying over time) noise, while otherportion of column noise may be temporal noise. Column noise reductionprocess 1000 according to various embodiments may handle spatial and 1/fnoise separately from temporal noise.

Thus, at block 1020, spatial and 1/f column noise may be reduced in eachtype of difference frame. For example, if difference frames 910 includefour types (e.g., even-down, odd-down, even-up, and odd-up) ofdifference frames, operations to reduce spatial column noise may beperformed separately on the four types of difference frames so that eachof the four types may have a respective corresponding spatial columnnoise correction terms associated with it. In various embodiments,spatial column noise may be estimated and appropriate correction termsmay be obtained using appropriate spatial column noise reductiontechniques. For example, in one or more embodiments, spatial columnnoise may be estimated and reduced in difference frames according to thetechniques disclosed in U.S. Pat. No. 8,208,026 entitled “Systems andMethods for Processing Infrared Images” and U.S. patent application Ser.No. 14/029,716 entitled “Row and Column Noise Reduction in ThermalImages,” which are incorporated herein by reference.

At block 1040, temporal column noise may be reduced for each type ofdifference frame. In some embodiments, temporal column noise in eachtype of difference frame may be estimated by comparing temporal changesin a particular column with temporal changes in neighboring columns.Temporal changes may be measured by determining changes in signal levelbetween a current and a previous difference frame of same type in asequence of difference frames 910. In some specific examples, temporalchanges in a particular column may be compared with temporal changes intwo (one on each side) or four (two on each side) neighboring columns.

In this regard, the inventors of the present disclosure have observedthat temporal changes in difference frames due to an actual scene highlylikely affects neighboring columns (e.g., temporal changes arecorrelated among neighboring columns). Thus, in one or more embodiments,temporal changes in an average signal level of a column is estimated astemporal column noise (e.g., not due to temporal changes in the actualscene) if it is uncorrelated with those of neighboring columns. In oneor more embodiments, temporal column noise estimated for a particularcolumn in such a way may be reduced by applying an infinite impulseresponse (“IIR”) filter having a damping factor that is inverselyproportional to the degree of correlation with neighboring columns interms of temporal changes. In other embodiments, other appropriatefilters may be used that have their filtering strengths adjusted inresponse to the degree of correlation with neighboring columns in termsof temporal changes.

Thus, by performing process 1000 according to one or more embodiments atblock 920, spatial, 1/f, and/or temporal noise may be estimated andreduced in difference frames 910. It should be noted that operations ofprocess 1000 may be performed in any desired order. That is, temporalcolumn noise may be reduced before spatial column noise, or vice versa,depending on embodiments.

Returning to process 900, at block 930, the even and the odd differenceframes may be fused or combined into a difference image havingdifference rows for all pairs of adjacent active bolometer rows. Asdiscussed above in connection with FIGS. 6A and 7A-7D, an even-downdifference frame and an odd-down difference frame, or an even-updifference frame and an odd-up difference frame, may each capture evendifference rows and odd difference rows, respectively, consecutively inan interlaced manner. Thus, for embodiments in which two types ofdifference frames (e.g., an even-down difference frame and an odd-downdifference frame together, or an even-up difference frame and an odd-updifference frame together) are captured, the even and the odd differenceframes may be combined or fused by taking even difference rows from aneven difference frame and taking odd difference rows from an odddifference frame.

For embodiments in which three or four types of difference frames arecaptured, difference rows in the combined or fused (e.g., composite)difference images may be obtained by averaging or accumulatingcorresponding difference rows from different types of difference frames.For example, even difference rows from an even-down difference frame maybe averaged or accumulated with corresponding even difference rows froma corresponding even-up difference image frame, and/or odd differencerows from an odd-down difference frame may be averaged or accumulatedwith corresponding odd difference rows from a corresponding odd-updifference image frame. Similarly, for embodiments in which progressivedifference frames are captured by bolometer circuit 500 or 800,corresponding difference rows may be averaged or accumulated.

In some embodiments, different types of consecutively captureddifference frames 910 may be aligned or registered prior to combining orfusing, depending on the amount of motion between 910 different framesand/or the thermal time constant (e.g., an inherent response time) ofactive bolometers 502. For example, if there is too much motion betweenconsecutive difference frames 910 and/or if active bolometer 502 is fastto respond to scene changes, consecutive difference frames 910 may bealigned or registered according to conventional image registrationtechniques, so that difference frames 910 may be more accurately fusedor combined with a consistent field-of-view.

Thus, after block 930, difference frames 910 are fused or combined intocomposite difference images that comprise all difference rows (e.g.,both even and odd rows) for all pairs of adjacent active bolometer rows.In some embodiments, at block 940, further noise reduction operationsmay be performed on such composite difference images in a differencedomain. An example noise reduction process 1100 that may be performed atblock 940 is illustrated according to an embodiment of the disclosure,with reference also to FIG. 11.

At block 1120, spatial and 1/f row noise may be reduced in suchcomposite difference images. As understood in the art, row noise is atype of noise that may be explained by variations in per-row circuitryand manifest as horizontal stripes (e.g., effecting an entire row) inimages, and may include spatial, 1/f, and/or temporal components. Rownoise may be handled in composite difference images, rather thanseparately on each type of difference frame, since all differencesignals in each difference row are effectively affected by same noisefor the row. In various embodiments, spatial row noise may be estimatedand appropriate correction terms may be obtained using appropriatespatial row noise reduction techniques. For example, in one or moreembodiments, spatial row column noise may be estimated and reduced incomposite difference images according to the techniques disclosed inU.S. Pat. No. 8,208,026 and U.S. patent application Ser. No. 14/029,716previously reference herein.

At block 1140, temporal row noise may be reduced in the compositedifference images. Temporal row noise may be estimated and filtered bycomparing temporal changes in each row with those of its neighbors, in amanner similar to estimating filtering temporal column noise discussedabove, but appropriately modified to handle rows rather than columns.

At block 1160, spatial per-pixel noise may be reduced in the compositedifference images. As understood in the art, per-pixel noise maycomprise spatially correlated or structured noise (also referred to asfixed-pattern noise) due to variations in active bolometers 502, opticalelements, mechanical elements, or other variations that affect responsecharacteristics of each active bolometer 502. In various embodiments,such spatial per-pixel noise may be estimated and appropriate correctionterms may be obtained using appropriate spatial column noise reductiontechniques. For example, in one or more embodiments, spatial columnnoise may be estimated and reduced in difference frames according to thetechniques disclosed in U.S. Pat. No. 8,208,026 entitled “Systems andMethods for Processing Infrared Images” and U.S. patent application Ser.No. 14/029,716 entitled “Row and Column Noise Reduction in ThermalImages,” which are incorporated herein by reference.

Per-pixel noise may also comprise spatially uncorrelated temporal noisethat randomly occurs in images (e.g., as white noise). Thus, at block1180, temporal per-pixel noise may be estimated and reduced usingappropriate temporal per-pixel noise (or random noise) reductiontechniques. For example, in one or more embodiments, temporal per-pixelnoise may be reduced by applying the techniques disclosed in U.S. patentapplication Ser. No. 13/943,035 entitled “Methods and Systems forSuppressing Noise in Images,” which is incorporated herein by reference.It should be noted that operations of process 1100 may be performed inany desired order. That is, temporal noise may be reduced before spatialnoise, and/or per-pixel noise may be reduced before row noise, dependingon embodiments.

Returning to process 900, after optionally reducing various types ofnoise in a difference domain at blocks 920 and/or 940, compositedifference images may be reconstructed into images in a direct imagedomain at block 950. As discussed above, in direct images, each pixelvalue may be indicative of the intensity of IR radiation received ateach detector, rather than a difference of intensity levels betweenadjacent rows. Thus, for example, direct images can be presented (e.g.,on a display) as user-viewable thermal images (e.g., thermograms) forviewing and easy understanding by a human user, with or without furtherprocessing. For applications such as video/image analytics or otherimage processing applications where viewing by a human user's is notneeded, process 900 may end prior to block 950.

In various embodiments, difference images may be reconstructed intodirect images by integrating all difference rows from top to bottom, oralternatively from bottom to top. In one or more embodiments,integrating all difference rows from top to bottom, or vice versa, mayinvolve obtaining a cumulative sum of the difference rows from top tobottom or from bottom to top. For embodiments in which the first and/orthe last difference rows may be absolute measurement rows comprisingsignals measured in absolute terms (e.g., via comparison with areference provided by blind bolometer 534 as discussed above withrespect to FIGS. 5A-5D, 6A, and 7A-7D), storing a cumulative sum as eachnew row in a reconstructed image from the top or from the bottomdifference row produces a direct image where all pixels are indicativeof incident IR intensity relative to a reference provided by acorresponding row of blind bolometers 534.

In other embodiments without absolute measurement rows, reconstructingdirect images from difference images may involve statistical methods tocorrect column offsets. For example, in one embodiment, a mean value ofoffsets in a reconstructed image may be calculated, and the offset ofeach column of the reconstructed image may be corrected based on themean offset value to approximate a direct image. In another embodiment,vertical gradients of columns (e.g., in a difference image orreconstructed image) may be obtained and compared between neighboringcolumns to correct column offsets. More specifically, rows that exhibitsimilar vertical gradients in the neighboring columns likely correspondto a same scene feature (e.g., road, sky, building, or othersubstantially uniform features in a scene captured by activebolometers), and therefore the neighboring columns may be normalized toapproximate true signal levels by adjusting the column offsets of theneighboring columns such that column to column differences are minimizedfor such rows corresponding to a same scene feature. In this sense, theadjustment of column offsets according to such an embodiment may beviewed as maximizing smoothness between neighboring columns for areaswith similar gradients. In embodiments where column difference signalscan be obtained as discussed above in connection with FIG. 6B, pixel orcolumn offsets between neighboring columns in a reconstructed image(whether absolute measurement rows were used or not) may be normalizedor otherwise adjusted based on the actual measured differences inincident IR radiation intensity between the neighboring columns asrepresented by the column difference signals for more accuratereconstruction.

In some embodiments, after reconstruction of difference images intodirect images, various operations to reduce one or more types of noiseintroduced or otherwise present in the direct images may be reduced atblock 960. FIG. 12 illustrates a noise reduction process 1200 that maybe performed on the reconstructed direct images at block 960 to reduceone or more types of noise, in accordance with an embodiment of thedisclosure.

At block 1210, frame bias noise may be estimated and reduced in directimages reconstructed from difference images. Conventionally, frame biasnoise is exhibited as a fixed offset that is uniform or substantiallyuniform over an entire image plane (e.g., over an entire frame of image)due to characteristics of a bolometer circuit. Such a uniform offset maybe ignored in difference images or in directed images that are captureddirectly with a bolometer circuit without differencing, since it simplyshifts an entire image plane (e.g., increases or decreases signal levelsof an entire image) by some uniform amount. However, for direct imagesthat are reconstructed from difference images, such a uniform offset indifference images may result in tilting of the reconstructed directimages (e.g., signals gradually increasing or decreasing from top tobottom rows) because of the integration or cumulative summation ofdifference rows in the reconstruction of the direct images.

In various embodiments, frame bias noise in direct images reconstructedfrom difference images may be estimated by comparing an extra absolutemeasurement row (e.g., an absolute measurement row that is not astarting row in the reconstruction) of the difference images with acorresponding end row of the direct images. Since the end row of areconstructed direct image should comprise substantially same signals(e.g., same data values) as the extra absolute measurement row withoutframe bias noise, an average difference of the signals between the endrow of the reconstructed direct image and the extra absolute measurementrow over all columns may be used as an estimate of a frame tilt for thecurrent direct image. According to one or more embodiments, the averagedifferences between the end row of the reconstructed direct image andthe extra absolute measurement row may be obtained and further averagedfor multiple direct images to provide an estimate of a fixed (e.g.,static or slowly varying) tilt, in other words, frame bias noise, indirect images.

In some embodiments, frame bias noise may be reduced in reconstructeddirect images by subtracting a per-pixel contribution of the estimatedframe bias noise (e.g., obtained by dividing the estimated frame biasnoise by the number of rows per column) from all pixels of thedifference images. In other embodiments, frame bias noise may be reducedin reconstructed direct images by subtracting a vertical ramp that isbased on the estimated frame tilt from the reconstructed direct images.For example, the vertical ramp may be based on a time averaged mean ofthe estimated frame tilt to generate varying signal levels depending onthe row position.

Depending on embodiments, frame bias noise may be reduced in the directimages that are used to estimate the frame bias noise, subsequentlyobtained direct images, or both. For example, the estimated frame biasnoise may be stored in memory and applied to reduce frame bias noise insubsequently obtained reconstructed direct image, according to someembodiments.

At block 1220, frame bounce noise may be estimated and reduced in directimages reconstructed from difference images, in some embodiments. Framebounce noise is a time-varying offset (if in difference images) or tilt(if in reconstructed direct images) that may vary from image to image(e.g., image frame to image frame), whereas frame bias noise is staticor slowly varying offset or tilt as discussed above. Thus, in one ormore embodiments, frame bounce noise may be obtained as a per-frameresidual frame tilt in each of the multiple direct images afterestimating and subtracting frame bias noise from the multiple directimages. The per-frame residual frame tilt may then be removed in adifference domain (e.g., by subtracting a per-pixel offset) or in adirect image domain (e.g., by subtracting a vertical ramp) to reduceframe bounce noise for each corresponding direct image, in a similarfashion as reducing frame bias noise discussed above for block 1210.

At block 1230, spatial column and pixel noise (fixed pattern noise) maybe estimated and reduced in in direct images reconstructed fromdifference images, according to one or more embodiments. As discussedabove with processes 1000 and 1100, spatial column and pixel noise maybe reduced in a difference domain prior to reconstruction. However,there may be some residual spatial column and pixel noise (residualfixed pattern noise) in difference images, which in the correspondingreconstructed direct images may be exhibited as erroneous column tilts.

Thus, in one or more embodiments, such erroneous tilts due to residualspatial (or fixed pattern) noise may be identified by comparing one ormore extra absolute measurement rows for each column with correspondingone or more reconstructed (e.g., integrated or cumulatively summed) rowsof the each column. An erroneous tilt of the column may be detected ifthe one or more extra absolute measurement rows differ from thecorresponding one or more reconstructed rows, and residual spatial (orfixed pattern) noise may be detected if the difference averaged overmultiple images is substantially non-zero (e.g., larger than a specifiedthreshold), according to one or more embodiments. If residual spatial(or fixed pattern) noise is detected in this way, it may be reduced byadjusting each column in difference images or by subtracting anappropriate vertical ramp from each column in reconstructed directimages.

At block 1240, residual noise in direct images may be estimated toadjust various noise reduction parameters, according to one or moreembodiments. Residual noise may be estimated from direct images with orwithout one or more of the noise reduction operations discussed abovefor blocks 1210-1230 applied thereto, depending on embodiments. Invarious embodiments, residual noise may be estimated by comparing thesignals (e.g., data values) at one or more extra absolute measurementrows of difference images with the signals at corresponding one or morerows of corresponding reconstructed direct images. For example, in oneembodiment, differences between one or more extra absolute measurementrows and corresponding reconstructed rows may be determined for allcolumns, and the standard deviation of such differences may be used as ametric for the amount of residual noise still present.

In one or more embodiments, the standard deviation of the differences orother metric determined based on the comparison with the one or moreextra absolute measurement rows may be used to adjust noise reductionparameters. For example, if the standard deviation is large or othermetrics indicate that a large amount of residual noise is still present,various parameters for one or more of the further noise reductionoperations on reconstructed direct images discussed below for blocks1260 and 1270, and/or various parameters for one or more of the noisereduction operations in a difference domain discussed above forprocesses 1000 and 1100, may be adjusted (e.g., by increasing filterstrengths) so that more aggressive noise reduction may be applied todifference frames, difference images, and/or reconstructed directimages.

At block 1250, noise reduction parameters may be adjusted for one ormore portions (e.g., local areas) of reconstructed direct images,according to one or more embodiments. In this regard, the inventors ofthe present disclosure have observed, through various experimentsconducted in connection with the disclosure, that some local areas(e.g., middle rows of reconstructed direct images when absolutemeasurement rows corresponding to top and bottom rows are provided) ofreconstructed direct images exhibit more noise than other areas. Thus,in one or more embodiments, such noisy local areas may be identified andnoise reduction parameters for the identified local areas may be furthertuned to reduce noise.

For example, in one embodiment, local smoothness (e.g., variation insignal levels for a local area) may be determined and compared betweendifference images and corresponding reconstructed direct images. If anarea in a difference domain is smooth (e.g., small variations in sampleddifferences) but the corresponding area in a direct image domain is notsmooth, it may be determined that the specific area needs furtherfiltering, and more aggressive filtering may be applied to the specificarea in reconstructed direct images by adjusting noise reductionparameters.

At block 1260, further noise reduction operations may be performed ondirect images reconstructed from difference images, according to one ormore embodiments. For example, various spatial and temporal, column/rowand per-pixel noise reduction techniques previously referenced herein inconnection with processes 1000 and 1100 may be appropriately modifiedand performed on reconstructed direct images. As discussed above, thevarious noise reduction operations may be tuned for more effective noisereduction by applying the noise reduction parameters as adjusted throughoperations of blocks 1240 and 1250.

At block 1270, multiple reconstructed direct images may be combinedusing an IIR or finite impulse response (“FIR”) filter to further reducetemporal noise in reconstructed direct images. Filter strengths andother parameters for the IIR or FIR filter may be based on theparameters adjusted through operations of blocks 1240 and/or 1250.

Returning to FIG. 9, after performing noise reduction on and usingreconstructed direct images at block 960 according to example noisereduction process 1200 of FIG. 12, reconstructed direct images 970 maybe provided with noise therein further reduced. Therefore, according toembodiments of the disclosure discussed above with reference to FIGS.5A-5D, 6A-6B, 7A-7D, and 8-12, difference images may be obtained usingbolometer circuit 500 or 800 with various beneficial features, andreconstructed into direct images having various types of noiseeffectively removed therefrom. More specifically, for example, bolometercircuit 500 or 800 advantageously provides high scene dynamics whilereducing the ill effects of self-heating and variations in componentcharacteristics without the added complexity, size, and cost ofconventional bolometer circuits as discussed above, while processes 900through 1200 advantageously utilizes absolute measuring rows toaccurately reconstruct difference images into direct images, toeffectively reduce various types of noise (e.g., frame bias noise, framebounce noise, and residual fixed pattern noise), and to adjust variousnoise reduction parameters. Furthermore, processes 900 through 1200advantageously identify and apply noise reduction operations that areeffective for difference frames, difference images, and reconstructeddirect images, respectively.

Turning to FIG. 13, a thermal imaging module 1300 that may compriseembodiments of bolometer circuit 300, 400A-D, 500, or 800 is illustratedin accordance with an embodiment of the disclosure. Also illustrated inFIG. 13 is a host device 1302 in which thermal imaging module 1300 maybe configured to be implemented, in accordance with an embodiment of thedisclosure.

Thermal imaging module 1300 may be implemented, for one or moreembodiments, with a small form factor and in accordance with wafer levelpackaging techniques or other packaging techniques. In this regard,thermal image module 1300 may comprise a housing 1304 that enclosesbolometer circuit 300, 400A-D, 500, or 800 and other components forgenerating (e.g., capturing) thermal image data representing incident IRradiation 201 from a scene 1350, according to one or more embodiments.Further in this regard, thermal image module 1300 may comprise anoptical element 1306 such as an IR-transmissive lens configured totransmit and focus incident IR radiation 201 on a FPA of bolometercircuit 300, 400A-D, 500, or 800. In some embodiments, these and otheraspects of thermal imaging module 1300 may be implemented according tovarious techniques for providing small form factor infrared imagingmodule disclosed for example in U.S. patent application Ser. No.14/101,258 entitled “Infrared Camera System Architectures,” which isincorporated herein by reference in its entirety.

In one embodiment, host device 1302 may be a small portable device, suchas a mobile telephone, a tablet computing device, a laptop computingdevice, a personal digital assistant, a visible light camera, a musicplayer, or any other appropriate portable device. In this regard,thermal imaging module 130 may be used to provide thermal imagingfeatures to host device 1302. For example, thermal imaging module 1300may be configured to capture, process, and/or otherwise manage thermalimages and provide such thermal images to host device 1302 for use inany desired fashion (e.g., for further processing, to store in memory,to display, to use by various applications running on host device 1302,to export to other devices, or other uses).

In other embodiments, host device 1302 may be other types of electronicdevices configured to receive thermal imaging module 1300 and to utilizethermal images provided by thermal imaging module 1300 for particularapplications. For example, host device 1302 may represent a fixedly oradjustably mounted (e.g., to provide pan-tilt-zoom features)surveillance camera, vehicle electronics (e.g., in sensor systems forautomobiles, aircrafts, and water vessels), or other non-portableelectronic devices. In another example, host device 1302 may include adevice attachment configured to receive thermal imaging module 1300 toprovide thermal imaging capabilities, and implemented according tovarious techniques disclosed in International Patent Application No.PCT/US2013/062433 entitled “Device Attachment with Infrared ImagingSensor,” which is incorporated herein by reference in its entirety.

In various embodiments, host device 1302 may include a socket 1310, ashutter 1312, a processor 1314, a memory 1316, a display 1318, motionssensors 1320 and/or other components 1322. Socket 1310 may be configuredto receive thermal imaging module 1300 as identified by arrow 1311, andshutter 1312 may be selectively positioned over socket 1310 (e.g., asidentified by arrows 1313) while thermal imaging module 1300 isinstalled therein.

Processor 1314 may be implemented as any appropriate processing device(e.g., logic device, microcontroller, processor, application specificintegrated circuit (ASIC), or other device) that may be used by hostdevice 1302 to execute appropriate instructions, such as softwareinstructions provided in memory 1316 (e.g., transferred or downloadedfrom a machine readable medium 1322 storing non-transitory softwareinstructions). As discussed above, in some embodiments, processor 1314may be configured to perform at least part of the various operations ofprocesses 900 through 1200, which may be encoded as software instructionprovided in memory 1316. For example, depending on embodiments,processor 1314 may be configured perform at least part of processes 900through 1200 independent of or in conjunction with processor or logicdevice 340 of bolometer circuit 500 or 800.

Motion sensors 1320 may be implemented by one or more accelerometers,gyroscopes, or other appropriate devices that may be used to detectmovement of host device 1302. Motion sensors 1320 may be monitored byand provide information to processor 1314 or processor/logic device 340to detect motion.

Display 1318 may be used to display captured and/or processed thermalimages and/or other images, data, and information. Motion sensors 1320may be implemented by one or more accelerometers, gyroscopes, or otherappropriate devices that may be used to detect movement of host device1302. Motion sensors 1320 may be monitored by and provide information toprocessor 1314 or processor/logic device 340 to detect motion. Othercomponents 1322 may be used to implement any features of host device1302 as may be desired for various applications (e.g., a visible lightcamera or other components).

Therefore, bolometer circuit 300, 400A-D, 500, or 800 thatadvantageously permits low cost, small footprint, and high performancethermal imaging as discussed above may be provided in a small formfactor thermal imaging module 1300 and integrated into various types ofhost devices 1302 for thermal imaging applications.

Where applicable, various embodiments provided by the present disclosurecan be implemented using hardware, software, or combinations of hardwareand software. Also where applicable, the various hardware componentsand/or software components set forth herein can be combined intocomposite components comprising software, hardware, and/or both withoutdeparting from the spirit of the present disclosure. Where applicable,the various hardware components and/or software components set forthherein can be separated into sub-components comprising software,hardware, or both without departing from the spirit of the presentdisclosure. In addition, where applicable, it is contemplated thatsoftware components can be implemented as hardware components, andvice-versa.

Software in accordance with the present disclosure, such asnon-transitory instructions, program code, and/or data, can be stored onone or more non-transitory machine readable mediums. It is alsocontemplated that software identified herein can be implemented usingone or more general purpose or specific purpose computers and/orcomputer systems, networked and/or otherwise. Where applicable, theordering of various steps described herein can be changed, combined intocomposite steps, and/or separated into sub-steps to provide featuresdescribed herein.

Embodiments described above illustrate but do not limit the invention.It should also be understood that numerous modifications and variationsare possible in accordance with the principles of the invention.Accordingly, the scope of the invention is defined only by the followingclaims.

What is claimed is:
 1. A bolometer circuit, comprising: a substrate; anactive bolometer configured to receive external infrared (IR) radiationand substantially thermally isolated from the substrate; a firstresistive load, wherein the active bolometer and the first resistiveload are configured to be connected in series in a bolometer conductionpath from a supply voltage node to a common voltage node; an amplifiercircuit comprising an operational amplifier (op-amp) having a firstinput coupled to a node in the bolometer conduction path between thefirst resistive load and the active bolometer; and a variable voltagesource coupled to a second input of the op-amp, wherein the variablevoltage source is configured to provide a reference voltage level,wherein the variable voltage source comprises a reference conductionpath comprising a blind bolometer and a second resistive load configuredto be connected in series from the supply voltage node to the commonvoltage node, wherein the blind bolometer is shielded from the externalIR radiation, thermally isolated from the substrate, and configured totrack self-heating of the active bolometer, wherein the amplifiercircuit is configured to generate a current flow to the amplifiercircuit in response to a resistance change of the active bolometer dueto the external IR radiation by maintaining the reference voltage levelat the first input of the op-amp, and wherein the amplifier circuit isfurther configured to convert the current flow into an output voltage atan output of the op-amp that is indicative of an intensity of theexternal IR radiation received at the active bolometer.
 2. The bolometercircuit of claim 1, wherein: the first resistive load comprises athermally shorted bolometer that is thermally shorted to the substrateto operate as a temperature-compensated load for the bolometerconduction path; a resistance of the first resistive load is similar toa resistance of the active bolometer; the amplifier circuit isconfigured to adjust a voltage offset at the first input of the op-amp,the second input of the op-amp, or both in response to offset adjustmentbits, so as to adjust a bias across the active bolometer; the bolometercircuit is configured to operate in a normal mode or in a low-power modebased on selective opening or closing of a plurality of switchesassociated with the bolometer conduction path and the amplifier circuit;in the normal mode, the output of the op-amp provides the output voltageindicative of the intensity of the external IR radiation received at theactive bolometer; and in the low-power mode, the output of the op-amp isdriven to a predetermined voltage level and the op-amp, the bolometerconduction path, or both are disconnected from power.
 3. The bolometercircuit of claim 1, further comprising a low-pass filter (LPF) coupledto the output of the op-amp to filter the output voltage to reducehigh-frequency noise, wherein: the amplifier circuit comprises aresistive gain coupling the output of the op-amp to the first input ofthe op-amp to configure the amplifier circuit as a feedback amplifier;the resistive gain comprises a thermally shorted bolometer that isthermally shorted to the substrate to operate as atemperature-compensated gain for the amplifier circuit; the amplifiercircuit is configured to maintain, in response to the reference voltagelevel, a bias voltage across the active bolometer and a load biasvoltage across the first resistive load via the first input of theop-amp coupled to the node in the bolometer conduction path; the currentflow to the amplifier circuit is generated in response to a differencebetween a current generated by the load bias voltage being applied tothe first resistive load and a current generated by the bias voltagebeing applied to the active bolometer that exhibits the resistancechange due to the external IR radiation; and the amplifier circuit isconfigured to generate the output voltage in response to the currentflow flowing through the resistive gain.
 4. The bolometer circuit ofclaim 1, wherein: the second resistive load has a similar resistance asthe first resistive load; and the reference voltage level provided bythe variable voltage source is provided from a node in the referenceconduction path between the blind bolometer and the second resistiveload, such that the current flow to the amplifier circuit is adjusted tocompensate for the self-heating of the active bolometer.
 5. Thebolometer circuit of claim 1, further comprising: a plurality of theactive bolometers arranged in a focal plane array (FPA) having columnsand rows; a plurality of column circuits each associated with a columnof the FPA and comprising a respective resistive load and a respectiveamplifier circuit; and a plurality of row-select switches associatedwith the respective active bolometers and configured to selectivelyconnect the respective active bolometers to the corresponding resistiveloads of the column circuits to generate output voltages for the activebolometers in a row-by-row fashion according to timing and controlsignals.
 6. The bolometer circuit of claim 1, wherein: the firstresistive load comprises a thermally shorted bolometer and a transistorconnected in series to operate as a current source that generates a loadcurrent to the active bolometer; the thermally shorted bolometer isthermally shorted to the substrate to operate as atemperature-compensated load for the bolometer conduction path; aresistance of the thermally shorted bolometer is larger than aresistance of the active bolometer; and the supply voltage node isconfigured to provide a supply voltage level that is higher than anominal voltage level applicable to generate the load current in anominal case of the thermally shorted bolometer having a resistancesimilar to the resistance of the active bolometer.
 7. The bolometercircuit of claim 6, wherein: the transistor comprises a metal oxidesemiconductor field effect transistor (MOSFET); the variable voltagesource is a first variable voltage source; and the bolometer circuitfurther comprises a second variable voltage source configured to providea variable voltage level to the gate of the MOSFET, the MOSFET beingconfigured to adjust the load current in response to the variablevoltage level.
 8. The bolometer circuit of claim 7, wherein: the firstresistive load comprises a first thermally shorted bolometer and a firstMOSFET; the second resistive load comprises a second thermally shortedbolometer and a second MOSFET connected in series to operate as acurrent source; the reference voltage level provided by the firstvariable voltage source is provided from a node in the referenceconduction path between the blind bolometer and the second resistiveload, such that the current flow to the amplifier circuit is adjusted tocompensate for the self-heating of the active bolometer; and thevariable voltage level from the second variable voltage source isprovided to the gates of the first and the second MOSFETs to adjustcurrents generated by the first and the second resistive loads.
 9. Thebolometer circuit of claim 6, wherein: the amplifier circuit furthercomprises: a capacitor coupling the output of the op-amp to the firstinput of the op-amp to configure the amplifier circuit as an integratingamplifier; a buffer connected to the node in the bolometer conductionpath; and a resistor connected to the first input of the op-amp, thebuffer and the resistor being connected in series to couple the node inthe bolometer conduction path to the first input of the op-amp; theamplifier circuit is configured to maintain the reference voltage levelat one end of the resistor connected to the first input of the op-amp;the other end of the resistor is configured to receive, via the buffer,a voltage level at the node in the bolometer conduction path set inresponse to the load current flowing through the active bolometer thatexhibits the resistance change due to the external IR radiation; thecurrent flow to the amplifier circuit is generated by the resistor dueto a difference between the reference voltage level at the one end andthe voltage level at the other end of the resistor; and the amplifiercircuit is configured to integrate the current flow by the capacitor togenerate the output voltage.
 10. A bolometer circuit, comprising: asubstrate; an active bolometer configured to receive external infrared(IR) radiation and substantially thermally isolated from the substrate;a resistive load, wherein the active bolometer and the resistive loadare configured to be connected in series in a bolometer conduction pathfrom a supply voltage node to a common voltage node; an amplifiercircuit comprising an operational amplifier (op-amp) having a firstinput coupled to a node in the bolometer conduction path between theresistive load and the active bolometer; and a variable voltage sourcecoupled to a second input of the op-amp, wherein the variable voltagesource is configured to provide a reference voltage level, wherein: theamplifier circuit is configured to generate a current flow to theamplifier circuit in response to a resistance change of the activebolometer due to the external IR radiation by maintaining the referencevoltage level at the first input of the op-amp; the amplifier circuit isfurther configured to convert the current flow into an output voltage atan output of the op-amp that is indicative of an intensity of theexternal IR radiation received at the active bolometer; the bolometercircuit is configured to operate in a normal mode or in a low-power modebased on selective opening or closing of a plurality of switchesassociated with the bolometer conduction path and the amplifier circuit;in the normal mode, the output of the op-amp provides the output voltageindicative of the intensity of the external IR radiation received at theactive bolometer; and in the low-power mode, the output of the op-amp isdriven to a predetermined voltage level and the op-amp, the bolometerconduction path, or both are disconnected from power.
 11. The bolometercircuit of claim 10, wherein: the resistive load is a first resistiveload; the variable voltage source comprises a reference conduction pathcomprising a blind bolometer and a second resistive load configured tobe connected in series from the supply voltage node to the commonvoltage node; the blind bolometer is shielded from the external IRradiation, thermally isolated from the substrate, and configured totrack self-heating of the active bolometer; the second resistive loadhas a similar resistance as the first resistive load; the referencevoltage level provided by the variable voltage source is provided from anode in the reference conduction path between the blind bolometer andthe second resistive load, such that the current flow to the amplifiercircuit is adjusted to compensate for the self-heating of the activebolometer; and in the low-power mode, the node in the referenceconduction path is disconnected from the second input of the op-amp andthe blind bolometer and the second resistive load are disconnected frompower.
 12. The bolometer circuit of claim 10, wherein the plurality ofswitches associated with the bolometer conduction path and the amplifiercircuit comprise: a first switch configured to connect or disconnect theresistive load to or from the supply voltage node; a second switchconfigured to connect or disconnect the active bolometer to or from thecommon voltage node according to control signals; a third switchconfigured to selectively short the output of the op-amp to the firstinput of the op-amp according to the control signals; a fourth switchconfigured to connect or disconnect the output of the op-amp to or fromthe supply voltage node to selectively drive the output of the op-amp toa supply voltage level at the supply voltage node according to thecontrol signals; and a fifth switch configured to connect or disconnectthe op-amp to or from power according to the control signals, whereinthe bolometer circuit is configured to operate in the low-power mode byopening the first, the second, and the fifth switches to disconnect theresistive load, the active bolometer, and the op-amp from power and byclosing the third and the fourth switches to drive the first input andthe output of the op-amp to the supply voltage level.
 13. The bolometercircuit of claim 10, further comprising: a plurality of activebolometers arranged in a bolometer focal plane array (FPA) havingcolumns and rows; a plurality of column circuits each associated with acolumn of the bolometer FPA and each comprising a respective resistiveload, a respective amplifier circuit, and respective switches associatedwith respective bolometer conduction paths and the respective amplifiercircuit; a control circuit configured to control each plurality ofswitches to operate the bolometer FPA in a normal imaging mode or alow-power detection mode; wherein in the normal imaging mode, allcolumns of the bolometer FPA operate in the normal mode; wherein in thelow-power detection mode, some columns of the bolometer FPA operate inthe normal mode while a remainder of the columns of the bolometer FPAoperate in the low-power mode; and wherein the control circuit isfurther configured to: detect a change in the external IR radiationusing those columns of the bolometer FPA that operate in the normal modewhile operating the bolometer FPA in the low-power detection mode; andswitch from the low-power detection mode to the normal imaging mode forthe bolometer FPA in response to detecting the change in the external IRradiation.
 14. The bolometer circuit of claim 13, wherein: the bolometercircuit is configured to capture a sequence of IR image framesrepresenting the external IR radiation; and in the low-power detectionmode, those columns that operate in the normal mode are selected in around-robin manner from all columns of the bolometer FPA as each IRimage frame is captured, so as to reduce a burn-in effect on the columnsof the bolometer FPA.
 15. A method of generating an output signal in abolometer circuit, the method comprising: selectively connecting anactive bolometer to a first resistive load in series to form a bolometerconduction path from a supply voltage node to a common voltage node,wherein the active bolometer is configured to receive external infrared(IR) radiation and substantially thermally isolated from a substrate towhich the active bolometer is attached; providing a reference voltagelevel to an operational amplifier (op-amp) that has a first inputcoupled to a node in the bolometer conduction path between the firstresistive load and the active bolometer, wherein the reference voltagelevel is received via a second input of the op-amp, wherein theproviding of the reference voltage level comprises compensating thereference voltage level for self-heating of the active bolometer bygenerating the reference voltage level using a reference conduction pathcomprising a series connection of a second resistive load and a blindbolometer that tracks the self-heating of the active bolometer, whereinthe blind bolometer is shielded from the external IR radiation andthermally isolated from the substrate; generating a current flow to theop-amp in response to a resistance change of the active bolometer due tothe external IR radiation by maintaining the reference voltage level atthe first input of the op-amp; and converting the current flow into anoutput voltage at an output of the op-amp that is indicative of anintensity of the external IR radiation received at the active bolometer.16. The method of claim 15, further comprising adjusting a voltageoffset at the first input of the op-amp, the second input of the op-amp,or both in response to offset adjustment bits, so as to adjust a biasacross the active bolometer, wherein: the first resistive load comprisesa thermally shorted bolometer that is thermally shorted to thesubstrate; and the method further comprises compensating, by thethermally shorted bolometer, the current flow for a temperature changein the substrate.
 17. The method of claim 15, further comprisinglow-pass filtering the output voltage to reduce high-frequency noise,wherein: the first input of the op-amp is coupled to the output of theop-amp via a resistive gain to form a feedback amplifier configuration;the resistive gain comprises a thermally shorted bolometer that isthermally shorted to the substrate; the method further comprisesmaintaining, in response to the reference voltage level, a bias voltageacross the active bolometer and a load bias voltage across the firstresistive load via the first input of the op-amp coupled to the node inthe bolometer conduction path; the generating of the current flowcomprises generating the current flow in response to a differencebetween a current generated by the load bias voltage being applied tothe first resistive load and a current generated by the bias voltagebeing applied to the active bolometer that exhibits the resistancechange due to the external IR radiation; and the converting of thecurrent flow comprises generating the output voltage in response to thecurrent flow flowing through the resistive gain.
 18. The method of claim15, wherein: the second resistive load has a similar resistance as thefirst resistive load; and the reference voltage level is provided from anode in the reference conduction path between the blind bolometer andthe second resistive load.
 19. The method of claim 15, wherein: thefirst resistive load comprises a thermally shorted bolometer and atransistor connected in series to operate as a current source thatgenerates a load current to the active bolometer; the thermally shortedbolometer is thermally shorted to the substrate to operate as atemperature-compensated load for the bolometer conduction path; aresistance of the thermally shorted bolometer is larger than aresistance of the active bolometer; and the method further comprisessupplying a supply voltage level that is higher than a nominal voltagelevel applicable to generate the load current in a nominal case of thethermally shorted bolometer having a resistance similar to theresistance of the active bolometer.
 20. The method of claim 19, wherein:the transistor comprises a metal oxide semiconductor field effecttransistor (MOSFET); and the method further comprises: compensating, bythe thermally shorted bolometer, the load current for a temperaturechange in the substrate; and providing a variable voltage level to thegate of the MOSFET to adjust the load current.
 21. The method of claim19, wherein: the first input of the op-amp is coupled to the output ofthe op-amp via a capacitor to form an integrating amplifierconfiguration; the node in the bolometer conduction path is coupled tothe first input of the op-amp via a buffer and a resistor connected inseries; the method further comprises: setting a voltage level at thenode in the bolometer conduction path in response to the load currentflowing through the active bolometer that exhibits the resistance changedue to the external IR radiation; receiving the voltage level by thebuffer to pass the voltage level to one end of the resistor; andmaintaining the reference voltage level at the other end of theresistor; the generating of the current flow comprises generating thecurrent flow in response to a difference between the voltage level atthe one end and the reference voltage level at the other end of theresistor; and the converting of the current flow comprises integratingthe current flow to the capacitor to generate the output voltage. 22.The method of claim 15, further comprising selectively operating thebolometer circuit in a normal mode or in a low-power mode by selectivelyopening or closing switches associated with the bolometer conductionpath and the op-amp, wherein: in the normal mode, the output of theop-amp provides the output voltage indicative of the intensity of theexternal IR radiation received at the active bolometer; and in thelow-power mode, the output of the op-amp is driven to a predeterminedvoltage level and the op-amp, the bolometer conduction path, or both aredisconnected from power.
 23. The method of claim 22, wherein: the secondresistive load has a similar resistance as the first resistive load; thereference voltage level is provided from a node in the referenceconduction path between the blind bolometer and the second resistiveload; and in the low-power mode, the node in the reference conductionpath is disconnected from the second input of the op-amp and the blindbolometer and the second resistive load are disconnected from power. 24.The method of claim 22, wherein: a plurality of active bolometers arearranged in a bolometer focal plane array (FPA) having columns and rows;a plurality of column circuits are each associated with a column of thebolometer FPA and each comprise a respective resistive load, arespective op-amp, and respective switches associated with respectivebolometer conduction paths and the respective op-amp; the method furthercomprises selectively operating the bolometer FPA in a normal imagingmode or a low-power detection mode; in the normal imaging mode, allcolumns of the bolometer FPA operate in the normal mode; in thelow-power detection mode, some columns of the bolometer FPA operate inthe normal mode while a remainder of the columns of the bolometer FPAoperate in the low-power mode; and the method further comprises:detecting a change in the external IR radiation using those columns ofthe bolometer FPA that operate in the normal mode while operating thebolometer FPA in the low-power detection mode; switching from thelow-power detection mode to the normal imaging mode for the bolometerFPA in response to detecting the change in the external IR radiation;capturing, using the bolometer FPA, a sequence of IR image framesrepresenting the external IR radiation; and while operating thebolometer FPA in the low-power detection mode, selecting those columnsthat operate in the normal mode in a round-robin manner from all columnsof the bolometer FPA as each IR image frames is captured, so as toreduce a burn-in effect on the columns of the bolometer FPA.